Test strips can have up to a 20% variation. The difference between 100 and 96 is statistically insignificant when it comes to testing, as you could test twice on the same prick and get those two readings. Unfortunately, test strips are not an exact science, but an imprecise general measurement that is the best we've got, yet still serves us well when viewed in perspective.
Also, strips are very sensitive to environmental issues. A little heat or moisture contamination can throw them off. I had a batch a couple of months ago that suddenly gave error readings. To my mind I had done nothing different - kept them in my nightstand as usual, closed the vial immediately after getting out a strip, etc. I called Accu-chek as I thought it was a bad batch. They had me punch whatever for diagnosis, and said it showed there had been a moisture issue. Only thing I could figure is after I washed my hands, they weren't =completely= dry and residual dampness had contaminated the whole lot. Accu-check sent me another batch as a goodwill gesture, and I'm now neurotically careful w/ my strips!
That said, your average could be inching up a tad - good that you're monitoring closely.