On what is this based? percentage of total votes? If I check Hillarys votes in Los Angeles County in 2016 it is 1,893,770. Biden is reported to have 2,980,478 now. So in absolute votes there is a big increase for Biden. However, Hillary took 71.4% of total votes and Biden only 71.15, so in terms of voting percentage Biden actually is slightly underperforming Hillary.

However in Dallas county, Biden got 65.14% of the votes vs Hillarys 61.1%, so there he performed better.