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).   

I doubt it. As BadBear said percentages move quickly when you have few reports and barely noticable when you have >1000 reports. IIRC hilariousandco is at >1900 reports probably more now.

Lets assume 97.0000% of these are correct, thats 1843 correct and 57 not. In order to reach 97.5% (I assume thats when 98% will be shown) he would have to make 2223 total correct reports vs 57 bad. Thats 380 good reports without fail.

For 79 posts with ~74.6835% assumed accuracy (59 good, 20 bad) you only need 1 good report (60/(60+20) = 75%)

Lets assume 97.0000% of these are correct, thats 1843 correct and 57 not. In order to reach 97.5% (I assume thats when 98% will be shown) he would have to make 2223 total correct reports vs 57 bad. Thats 380 good reports without fail.