Metal UHF RFID tag simulation

Hi to my friends

I am simulating a UHF RFID tag,layout is like:
///pet(0.5mm)
///metal(0.018mm)
///pet(4mm)
///metal(0.018)reflect layout
///pet(0.05)


Port setting:
P1:Internal port(14+j144)
P2:Ground ref(associate with p1)
S(1,1):
Fre:914Mhz
Imp:Z0*(1.013+j0.065)
Mag.(db):35.45

My question:
Frequency move to 885Mhz in really measure
Why?