The vote count turn is unsurprising since the in-person ballots could be quickly tallied while the mail-in ballots had to be hand-counted.

The reason the two types of balloting had such a difference in party-support is that the President in the Republican party had been telling his supporters not to use mail-in balloting because it would be rife with fraud. People took him at his word on that side of the political divide, so there's a resulting bias in the likely candidate supported by the mail-in ballots.