Since the beginning of the pandemic, supporters of Democratic nominee Joe Biden have shown a strong preference for mail-in voting. Most of President Trump’s supporters said they wanted to vote on Election Day. States count these different types of votes in very different ways.

As a result, in some states — particularly in Pennsylvania — Trump saw an early lead that is now narrowing as more ballots are counted.

This isn’t a sign of fraud or irregularities. Rather, it’s just a reflection of how states count votes. Some states process early ballots first, while others save them for last.

This phenomenon, known as the “blue shift,” is common in recent US elections and it’s a big reason why Trump, despite election law and common decency to the contrary, has argued that whoever appears to have won on Election Night should be crowned the winner.

That’s not the way it works, of course. Ballots in North Carolina and Pennsylvania can arrive in the days after Election Day as long as they have a Nov. 3 postmark.

In Pennsylvania, election officials couldn’t do anything with early ballots until Election Day. Some counties didn’t even pick them up until the day after Election Day

CNN