Different between P3 & P6 in update ???

Hello all, how are you 

I faced problem when i doing update for schedule, my problem is when I put progress data in schedule but in (P3) i had total float = -10 , but when put the same data on schedule (P6) , i have total float = -5, the base schedule was on (P3) and convert it to (P6), i think it must total float the same in P3 & P6 , the schedule is the same 

please advice