Wait a moment.

So you are saying that the official results are correctly allocating 74 million votes to Trump but the 81 million for Biden are a lie?

They are both a lie, Trump's is actually higher but his votes got shifted. The point isn't the vote accuracy, should be obvious, the point is the amount. Even at 74 million he still got the most votes ever. Not exactly a picture of defeat.

Why would they tell the truth about Trump's votes and then lie about Biden?

They didn't. Both totals are a lie. Trump probably got 80 mil+ and Biden likely got 60 mil, which is more in line with overall vote totals from previous years.

Why not give Trump a much lower vote count in the first place?

Because Trump's votes came in on election day, preventing any overt cheating. Here they needed to rely on the vote shifting , which was too small a percentage to overcome the large amount of votes he was getting.