Math. The more reports you have overall, the larger the spread between percentage points. If reports were rounded off to 100ths, like 96.57% you would see it going up more often. Also could be getting some bad reports occasionally, probably won't be enough to drop your percentage visibly at this point so you wouldn't know.  

Also, when you report something, it will immediately be added to your number of reports, however it won't factor into your percentage until dealt with by a moderator, and never will if the report remains in limbo (it happens).