het scatterbrain, you haven't taken difficulty changes into account. If you plot the coins you get in your bitcoin client against days, you'll see a slight curve. You can do one of two things:

• Calculate the expected payment per share for each difficulty (50/difficulty), add up the total shares for the period and multiply, and you'll get what you would have gotten at a fee-free pps pool
• Only average your results per difficulty period

Hope this helps.
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i realize i didn't account for difficulty. i wasn't exactly sure how or if it was applicable. there were 2 difficulties in my data and it went down by only 83000 when it changed; a pretty small amount compared to most of the other difficulty changes i've seen. small enough i thought to not really have an effect on how it would impact results if it was applicable. is that a poor assumption?

why exactly does difficulty need to be applied at all? how does it change the ratio of my actual earned BTC/day/Ghash/s to somone's BTC/day/Ghash/s who mines at a PPS pool?