duration calculation in p3& p5

hi
i had to export some progs in p3 for but when i looked at the durations in p3 it had changed
Let me explain in p5
say activity A start date & finish dates are
02/05/07 , 05/08/07.
but same activity A in p3 has start & finish dates
are 02/05/07, 02/08/07.
Calender is same in both,
the relationship remains the same.

then why does it calculate like that?

cheers
mangesh