DC和PT报的delay不一样是为啥啊？

PT报的一个cell的delay为0.396。DC报的一个cell的delay为0.948 PT好像用了wire_load_model DC用的Topographical flow
PT查的report为
pt_shell> report_delay_calculation -from TOP_Inst/B_Inst/C_Inst/U106/A0 -to TOP_Inst/B_Inst/C_Inst/U106/Y
****************************************
Report : delay_calculation
Design : digi_top
Version: E-2010.12
Date   : Sat Feb  2 18:53:21 2013
****************************************
From pin: TOP_Inst/B_Inst/C_Inst/U106/A0
To pin:   TOP_Inst/B_Inst/C_Inst/U106/Y
Main Library Units:  1ns  1pF  1kOhm
Library: 'sc7_logic013_base_hvt_ss_typical_max_0p90v_125c'
Library Units:  1ns  1pF  1kOhm
Library Cell: 'AOI211XLMTH'
arc sense:                       negative_unate
arc SDF condition:                A1==1'b1&&B0==1'b0&&C0==1'b0
arc type:                        cell
Units:  1ns  1pF  1kOhm
Rise Delay
   cell delay = 1.74221
     Table is indexed by
      (X) input_pin_transition = 0.788503
      (Y) output_net_total_cap = 0.00931459
     Relevant portion of lookup table:
                   (X)  0.3789      (X)  0.9276
  (Y)  0.0049      (Z)  1.1570      (Z)  1.3585
  (Y)  0.0094      (Z)  1.6048      (Z)  1.8061
     Z = A + B*X + C*Y + D*X*Y
     A =  0.5271           B =  0.3674
     C = 99.3881           D = -0.0485
  Z = 1.74221
  scaling result for operating conditions
  multiplying by 1 gives 1.74221
Fall Delay
   cell delay = 0.982899
     Table is indexed by
      (X) input_pin_transition = 1.69249
      (Y) output_net_total_cap = 0.0090039
     Relevant portion of lookup table:
                   (X)  0.9276      (X)  1.7824
  (Y)  0.0049      (Z)  0.6210      (Z)  0.8442
  (Y)  0.0094      (Z)  0.7589      (Z)  1.0288
     Z = A + B*X + C*Y + D*X*Y
     A =  0.2831           B =  0.2013
     C = 19.3658           D = 12.1218
  Z = 0.982899
  scaling result for operating conditions
  multiplying by 1 gives 0.982899
  Cell Delay
  rise:  1.74221
  fall:  0.982899
Rise delay = 1.74221
Fall delay = 0.982899
Units:  1ns  1pF  1kOhm
  Transition rise
   transition = 2.0725
     Table is indexed by
      (X) input_pin_transition = 0.788503
      (Y) output_net_total_cap = 0.00931459
     Relevant portion of lookup table:
                   (X)  0.3789      (X)  0.9276
  (Y)  0.0049      (Z)  1.3414      (Z)  1.3398
  (Y)  0.0094      (Z)  2.0960      (Z)  2.0935
     Z = A + B*X + C*Y + D*X*Y
     A =  0.5151           B = -0.0016
     C = 167.5672           D = -0.2992
  Z = 2.0725
  scaling result for operating conditions
  multiplying by 1 gives 2.0725
  Transition fall
   transition = 0.827269
     Table is indexed by
      (X) input_pin_transition = 1.69249
      (Y) output_net_total_cap = 0.0090039
     Relevant portion of lookup table:
                   (X)  0.9276      (X)  1.7824
  (Y)  0.0049      (Z)  0.5067      (Z)  0.6922
  (Y)  0.0094      (Z)  0.6742      (Z)  0.8640
     Z = A + B*X + C*Y + D*X*Y
     A =  0.1270           B =  0.2115
     C = 36.1315           D =  1.1213
  Z = 0.827269
  scaling result for operating conditions
  multiplying by 1 gives 0.827269
Rise transition = 2.0725
Fall transition = 0.827269
1
DC查的report为
dc_shell-topo> report_delay_calculation -from B_Inst/C_Inst/U106/A0 -to B_Inst/C_Inst/U106/Y

****************************************
Report : delay_calculation
Design : cry_top
Version: E-2010.12
Date   : Sat Feb  2 18:52:20 2013
****************************************
From pin:                         B_Inst/C_Inst/U106/A0
To pin:                           B_Inst/C_Inst/U106/Y
Main Library Units:  1ns  1pF  1kOhm
Operating Conditions: ss_typical_max_0p90v_125c   Library: sc7_logic013_base_hvt_ss_typical_max_0p90v_125c
Library: 'sc7_logic013_base_hvt_ss_typical_max_0p90v_125c'
Library Units:  1ns  1pF  1kOhm
Library Cell: 'AOI211XLMTH'
arc sense:                        negative_unate
arc type:                         cell
Rise Delay
   cell delay = 0.859097
     Table is indexed by
      (X) input_pin_transition = 0.33499
      (Y) output_net_total_cap = 0.0021403
     Relevant portion of lookup table:
                   (X)  0.0916      (X)  0.3789
  (Y)  0.0020      (Z)  0.7477      (Z)  0.8681
  (Y)  0.0049      (Z)  1.0350      (Z)  1.1570
     Z = A + B*X + C*Y + D*X*Y
     A =  0.5064           B =  0.4152
     C = 99.1719           D =  1.9170
  Z = 0.859097
  scaling result for operating conditions
  multiplying by 1 gives 0.859097
Fall Delay
   cell delay = 0.400411
     Table is indexed by
      (X) input_pin_transition = 0.559958
      (Y) output_net_total_cap = 0.0021403
     Relevant portion of lookup table:
                   (X)  0.3789      (X)  0.9276
  (Y)  0.0020      (Z)  0.3392      (Z)  0.5155
  (Y)  0.0049      (Z)  0.4251      (Z)  0.6210
     Z = A + B*X + C*Y + D*X*Y
     A =  0.1663           B =  0.2961
     C = 25.0283           D = 12.3294
  Z = 0.400411
  scaling result for operating conditions
  multiplying by 1 gives 0.400411
  Cell Delay
  rise:  0.859097
  fall:  0.400411
Transition calculations
  Transition rise
   transition = 0.871756
     Table is indexed by
      (X) input_pin_transition = 0.33499
      (Y) output_net_total_cap = 0.0021403
     Relevant portion of lookup table:
                   (X)  0.0916      (X)  0.3789
  (Y)  0.0020      (Z)  0.8556      (Z)  0.8560
  (Y)  0.0049      (Z)  1.3408      (Z)  1.3414
     Z = A + B*X + C*Y + D*X*Y
     A =  0.5124           B =  0.0005
     C = 167.7021           D =  0.4007
  Z = 0.871756
  scaling result for operating conditions
  multiplying by 1 gives 0.871756
  Transition fall
   transition = 0.335968
     Table is indexed by
      (X) input_pin_transition = 0.559958
      (Y) output_net_total_cap = 0.0021403
     Relevant portion of lookup table:
                   (X)  0.3789      (X)  0.9276
  (Y)  0.0020      (Z)  0.2968      (Z)  0.4030
  (Y)  0.0049      (Z)  0.4335      (Z)  0.5067
     Z = A + B*X + C*Y + D*X*Y
     A =  0.1107           B =  0.2361
     C = 55.1269           D = -20.7926
  Z = 0.335968
  scaling result for operating conditions
  multiplying by 1 gives 0.335968
1