And in 2020 Trump and Biden's votes together was just 66 percent of the legal voting population. So even with record voting thirty three percent of Adult American citizens choose not to vote.
So to answer your question. There is nothing shady about it, more to citizens choose to vote. That is all.
Also not all counties have the same number of people. Rural counties tend to have a far lower population than urban counties and so yes Biden could easily win less counties be get a higher number of votes.