Freebitco use parimutuel betting system. All other bookmaker use fixed odds system. As a result you will always see difference between freebitco and other sports betting site odds. Betting odds change in parimutuel betting system as long as bettors are placing bets on that match. Although early bettors get an advantages for time weight multiplier.
@hosseinimr93, Most of the sports betting website use a odds provider. They don't calculate it by themselves.