请教task为什么时间没有变化？

请教一下：
    在调用另外一个模块的posedge_task的task set_simple_out的时候，总是少了一个时钟
    而且打印的时间为
          start time--->time= 5
      pos=0 time=5
      pos=1 time=5
   不知道这个是什么原因？
   但是调用top_tb_posedge里面的task  my_set则没有这个问题。
module top_tb_posedge;
reg clk;
wire val;
wire [7:0]dout;
wire last_one;
reg val_new;
reg dout_new;
task my_set;
input [3:0] ll;
integer mm;
begin
   for(mm=0;mm<4;mm = mm +1)
    begin
      val_new=1'b1;
      dout_new=1'b1;
      $display(" my_sime %t at [%d] ",$time,mm);
      @(posedge clk);
    end
   $display(" my_sime %t ",$time);
   val_new =1'b0;
   @(posedge clk);
end
endtask
//reg [7:0]dout;
//reg val;
//reg last_one;
   posedge_task pt
      (.clk(clk),
       .dout(dout),
       .val(val),
       .last_one(last_one)
     );
   initial
     begin
        clk=0;
        forever #5 clk=~clk;
     end
  
   initial
      begin
        $display(" before start time--->time= %t\n",$time);
         @(posedge clk);
         $display(" start time--->time= %t",$time);
         pt.data_buffer[0]=1;
         pt.data_buffer[1]=2;
         pt.data_buffer[2]=3;
         pt.data_buffer[3]=4;
         //my_set(4);
         pt.set_dout_simple(4);
         #1000;
         $finish;
      end
   always @(last_one or dout or val)
   begin
      $display( " last_one=%d,dout=%d,val=%d,time=%t\n",last_one,dout,val,$time);
   end
endmodule
//////////////////////////////////////////////////////////////////
module posedge_task(clk,dout,val,last_one);
input clk;
output val;
output [7:0]dout;
output last_one;
reg [7:0]dout;
reg val;
reg last_one;
reg  data_buffer[0:255];
task set_dout_simple;
  input [3:0]element_num;
  integer pos;
  reg quit_now;
  
  begin
     for(pos=0;pos<4;pos=pos+1)
     begin
         $display(" ---> simple : pos=%d,time=%d \n",pos,$time)         ;
         dout=pos;
         val=1;
         @(posedge clk);
          //pos=0;
     end  
     val=0;
     @(posedge clk);
    
    end
  endtask
task set_dout;
input [3:0]element_num;
integer pos;
reg quit_now;
begin
    pos=0;
    $display(" --->pos=%d,time=%d \n",pos,$time);
    dout=data_buffer[pos];
    pos=pos+1;
    val=1'b1;
    if(element_num<=1)
       last_one=1'b1;
    else
       last_one=1'b0;
    quit_now=1'b0;
    
    while(quit_now==0)
    begin //{
        @(posedge clk);
        if(last_one)
          begin
            val=1'b0;
            quit_now=1'b1;
           end
        else
          begin
             val=1'b1;
             $display(" --->pos=%d,time=%d \n",pos,$time);
             dout=data_buffer[pos];
             pos=pos+1;
             if(element_num==1)
                last_one=1;
             element_num=element_num-1;              
          end            
    end//}
end
endtask
endmodule