each pool uses their own avg system. (thats the ppns /pps etc)    so when they number is guesses, it calculates based on a guess.
  the actual result is the only one that really matters.   ie: if pool says your going to find 10 coins in an hour, but u find 8 , thats still accurate.
 the next hour u might find 6, and then the next 3 hrs you find 12... and so on.

    Based your decisions on weekly rates, or at least daily. (full 24hrs)  some pools i have been on, dont start calculations until after 24hrs. (those seem to be the more honest setups, imo)