This doesn’t add up

Has anyone seen this analysis from the Tatum Report about how the number of votes cast nationwide in this election (i.e., the “popular vote”) is 12.5 million more than the number of people who voted, which means that Biden’s team either flipped or invented 12.5 million votes?  

The calculation goes like this:  

1. WaPo claims that voter turnout was the highest since the 19th century, with 66.2% of registered voters casting ballots.  They linked to a website that provides the number of  registered voters for each each state (each of a different reporting date, but all providing a number sometime in 2020, most in October or November).  
2. Gateway Pundit adds up all of the registered voters.   That number is 213.8 million.  
3. 66.2% (turnout) of 213.8 million registered voters is 141.5 million actual votes cast.  
4. DJT won 74 million votes.  141.5 minus 74 leaves only 67.5 million votes for Biden.  
5. Biden claims 80 million votes.  That’s a 12.5 million vote discrepancy.  
6. Therefore, Biden manufactured (and/or flipped) 12.5 million votes.  (The 12.5 million also no doubt includes a non-negligible number of ballots cast by people who were not eligible to vote, perhaps a large enough number to overturn the election if their votes are properly removed from the tallies—which is what most of the lawsuits have focused on.)  

Now, I’m not fully convinced of this math.  

For one thing, the site reporting the number of registered voters also reports the total population and calculates a percentage of the population that is registered.  That percentage is 65%.  Coincidence?  Does WaPo conflate these two very different percentages (percentage of population who registered vs. percentage of registered who voted)?  

Or is this really the smoking gun?  Not just a precinct here or there reporting twice as many ballots cast as eligible voters exist, but 12.5 million “overvotes” nationwide?  

What am I missing?