don't try and check the math, it will only make you crazy. If after a few days things don't make sense then question it. less than a day or two is too short term to get any accuracy.
+1
That's always the case - people expect a change made from them to make immediate difference, which just can't happen with mining, because it is 'chance dependant' - you may change something and have a bad luck for some time, but the change may end positive and the opposite - because of luck you may think that something worked better, but in fact you loose more with the time.
This is the reason why pool admins need some time to get the right formula for the vardiff and then the pool software will need some time get the right value for your speed, which in turn causes some variance in the reward earned, which will even out at the end, but may be far from the best in a single round
