They got back to me this morning. The answer is...
These are notes I bought on folio. I had forgotten that I ever bought notes on folio, but apparently I did, several years ago.
For loan ID 36784, the web shows $800 'cause that's the original amount loaned in this note. My statement says Principal Invested $781.22 because that's the par value of the note at the time I bought it. (some principal had been repaid) Finally, the notes.csv file says Amount Lent $756.55 because that's the amount I paid for the note on folio. This explains why the three numbers are different.
Would be nice if there was a data dictionary for the notes.csv file that said what the fields were supposed to mean.
So when calculating from a notes.csv file, I guess you shouldn't be surprised if it appears someone paid you back more than you lent, because the "amount lent" numbers there aren't actually what you lent.
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