But it's also undeniable that the average American leans left. That's why Republicans lose the popular vote in both chambers of congress and the presidency nearly every election. Their positions are unpopular, that's why they get down voted on "mainstream" sites where the user base is large enough to revert to the mean.
And campaign focus outside of urban cores would mean that the average American is even farther left than they appear to be.
Biden is farther right than the average Democrat because he needed to be to win. The structural advantages to Conservatives means the Democratic party needs to run centrist candidates.
In comparison, the same advantages mean the Republican party can still win with extreme right wing candidates. For example, Trump was the first president that never hit 50% approval rating but it was still high enough for him to win and do okay during his second campaign.