On a non-guldencoin related question:

How come my 24 hour payout amount seldom - if ever - matches the BTC/day per MH/s multiplied by my avg hash rate per that 24 hours.

If, for some illogical reason, BTC/day per MH/s multiplied by my avg hash rate per that 24 hours is not what I should be getting paid, what is the secret formula?