The reason is actually straight forward. In the quarterly report, about 33% of applications get rejected, but we know the trackers show approval % of 90+. This is because there is a good chunk of applications that get rejected right at the start. These are not shown in our data tracking, but it's captured in USCIS's quarterly data. If you takeout 75% of the rejected number and add to the "approved" numbers, should come out about right.
So unfortunately we are not underestimating.