Oh man of the past, for whom reality is scary, we here in the future are not afraid. What you think is a discrepancy is actually just a poor understanding of numbers.
See registered Democrats asked for 1941436 mail-in ballots and of those registered Democrats 1702623 returned them, and for simplicity's sake we're gonna assume they voted for Biden.
Now Biden got 1995691 mail in ballots total. This is a different number than 1702623, you are correct in that. However the numbers are not a count of the same thing.
One is the returned mail-in ballots of registered Democrats, the other is a total of state-wide, multiple parties returned mail-in ballots. We here in the future refer to this as "Two different things". We coined the term on the 15th I think, many days after the time-warp brought you to us, so you wouldn't have heard of "Two different things".