Help with some maths in assembly

Hi, I have this code which I am simulating using Proteus. I am running the PIC @ 4MHz so every instruction is 1uS.
Now my code runs a delay routine 0-255 then inside that routine another routine runs 0-255. Total delay should be 255 x 255 = 65025 mS. But the delay is somewhere between 180-200mS region. Can someone explain why this is happening please? Thanks

Code:

PAGE0 CLRF PORTD CLRF PORTB PAGE1 CLRF TRISD CLRF TRISB PAGE0 MOVLW 0 MOVWF COUNT1 MOVWF COUNT2 MOVWF COUNT3 MOVWF COUNT4 MAIN BSF RSET CALL DELAY250 BCF RSET CALL DELAY250 MOVWF COUNT1 MOVWF COUNT2 GOTO MAIN DELAY250 CALL DELAY500 INCFSZ COUNT1,f GOTO DELAY250 BCF STATUS,Z RETURN DELAY500 INCFSZ COUNT2,f GOTO DELAY500 BCF STATUS,Z RETURN END