After a little further investigation, it looks to me like the "Principal Balance" column is accumulating rounding errors over time.
Newer notes with only 1 or 2 payments, these numbers are the same.
Older notes with more than 2 payments, the numbers often no longer match.
After 3 or 4 payments they might be different by just a penny. But some of my older notes they're off by 10 or 15 cents.
Not sure if this "Principal Balance" number is used anywhere outside of this one page though, so it might not be important... but little errors like these always make me wonder how good the rest of their system is at keeping track of things.