Doing the math, Biden’s margin of about 25 points is more than 10 points lower than Clinton’s. This is more because Trump is doing better without third party candidates than Biden taking a lower share of the vote, though both seem to be occurring to some degree.
Biden’s weaker margin is occurring even as he is doing better overall. In the last two months, Biden’s margin over Trump overall is nearly double in these eight polls (9 points) what Clinton’s was among registered voters (5 points) in the final 2016 polls of registered voters.
Biden’s relative weakness with Hispanics extends back to the Democratic primary. In the caucuses and primaries where Hispanics made up at least 5% of primary voters or caucusgoers in the entrance and exit polls, Biden averaged 32% of their vote. He averaged 38% of the vote overall in these contests.
Biden is managing to do better than Clinton overall because of how he is doing with black and white voters.