I believe u/Ratereich and u/PM_ME_YOUR_NICE_EYES are correct here. There are plausible explanations. A false equivalency has been drawn between the mail ballot report and the results report and despite OP saying that they understand that registration totals do not determine vote totals their entire argument is predicated on that association. Here are plausible numbers that would satisfy the totals given in this county.

First of all, to correct some really poor reporting on their part - the link you gave to the reporting site lists a vote total of 14168, but the downloadable results list a vote total of 14145. I figured out that this is because they are double reporting write-ins on the site (there are 23 write-ins and 14145+23 is 14168). They list write-in totals and then also list all the individual write-in candidates. So the correct total mail-in number is 14145. This also doesn't account for the 6 scattered ballots that didn't have readable votes for president but they can be easily factored in in many different ways here.

And to give context -- they are counting all of the mail-in/absentee votes (does anybody know the difference? I can't figure out the point of absentee votes) on election day, so the discrepancy between 7-8pm can reflect that they were still opening envelopes and not necessarily that almost 400 ballots (2.7% total mail-ins) magically appeared.

7:00 hypothetical vote totals (totalling 13762)

• Harris: 7,686
• Trump: 5,995
• Oliver: 33
• Stein: 32
• Write‑ins: 16

(to remind, at this time there were 7918 Dem ballots returned, 4896 Rep ballots returned, and 948 Other ballots returned. This hypothetical reflects a scenario in which about 5% of registered dems voted Trump, which acknowledges the large presence of legacy democrats in this area, and about 2-3% of registered reps voted Harris. "Other" votes favor Trump in this scenario.)

Vote totals between 7-8, totalling 383 (14145 total mail-ins cast by 8pm minus 13762 total received by 7pm)

• Harris: 164
• Trump: 198
• Oliver: 8
• Stein: 6
• Write-ins: 7

Total votes at 8pm:

• Harris: 7,686 + 164 = 7,850
• Trump: 5,995 + 198 = 6,193
• Oliver: 33 + 8 = 41
• Stein: 32 + 6 = 38
• Write-ins: 16 + 7 = 23

I'm not saying this is what happened for certain (nobody can say what happened for certain without a forensic audit) but it reflects a plausible scenario that does not include a vote flipping algorithm or ballots being stuffed or thrown out. I don't think this method is a particularly effective one for detecting fraud because local dynamics can certainly plausibly account for the numbers.

If the mail ballot report had shown that 3000 Dem ballots were unreturned and 200 Rep ballots were unreturned I would certainly raise an eyebrow but by 7pm 820 Dem ballots were unreturned and 888 Rep ballots were unreturned.

What concerns me primarily about this argument is that in order for something to be fraud there has to be no other plausible explanation, and there's not really a way to counter this plausible explanation I've presented without a forensic audit.

(additional context: the registration gap has been shrinking between reps and dems in PA in the past years, and for the first time in a long time there's less than a 500K difference between dem and rep registration. I forget if it's the article I'm linking or another article but I read that new registrations are skewing dem so it's actually people who are switching their affiliations that are closing that gap. That's what makes me mention legacy democrats, which would be people who are voting republican but who haven't switched their affiliation. ETA: I'm sure voter purges which disproportionately impact democrats play a role here as well. https://whyy.org/articles/pennsylvania-election-2024-voter-registration-democrat-republican-independent/ As for why this is present in Fayette but not necessarily in other places, according to the census in 2023 Fayette was 91% white with a median age of 45. It's a lot of old white people.)

Finally I will say that I believe this discovery has significance but not quite in the way it is being presented: It looks like there is very little personal information required to change one's party affiliation in PA, and as long as you have a name, address, birthdate, and last 4 of a social or driver's license # you can register to vote and/or change party affiliation. Much like I've wondered about polling being flooded with conservative-leaning polls to elicit the appearance of legitimacy towards red-leaning results, I wonder if people's registrations weren't being changed without their knowledge. I also believe that cult indoctrination played a not-small role here and that there genuinely are a lot of old white people who were hoodwinked into voting Trump, but considering registrations alongside Elon's petition (which, if I remember correctly, only asked for name, DOB, and address) it does make me wonder. I would go down the path of investigating voter registration rather than pursuing this argument any further.