I think I remember reading something one time that if your use your card regularly and build a reasonable balance, the score goes up because you're showing responsible usage (obviously) but that balance has to carry over to the next billing cycle to be reported. If you pay it off before each bill and your balance due is always zero, then while the credit card company sees you are using their product, they have to report a zero balance to the bureaus and thus that shows as zero activity. This can lower your score if it was previously built up. I could be remembering incorrectly though.

I know mine goes up and down by a few points from time to time but it usually stays in the same general area. A lot of credit card companies are now offering a form of free credit monitoring so if you have one of those eligible cards you may want to check it out and see if it gives a detailed explanation.