Ah yes, you would get exact amount of shares only if you used low diff shares. Check this thread (I don't know which page, was long time ago) - we already had debate about this.
Keep in mind that our pool on the miners side allows miners to have diffs lower than what pool provides - they get paid for these shares but are not submitted to pool if being under pool diff.
Wait, you are saying that i'll get 20% less shares if i use a difficulty compatible with the mining speed? Thats simply unacceptable. And that is not the explanation since i use the same pool with the same setup since december and only yesterday this difference occured, so, thats not the explanation.
I m of course expecting small differences to occur, but 20% is unacceptable.