Looking at the poll numbers shown by realclearpolitics.com for 2020 Democratic Presidential Nomination. Specifically Biden's poll numbers. There seems to be two trend-lines for him instead of one. One hovers 29-34 and another distinctly lower between 22-26. To put it another way: since 7/1 he has 18 results in the 29-34 range and interspersed, you'll see the occasional blip going down to 22-26. Since 7/1 there are only 8 results in the 22-26 range. Going back further seems to show the same high/low pattern.
Something's happening here. There is a weird high versus low pattern in Biden's poll results; most of the numbers fall very close to either the high or the low with very few in between. There must be a different methodology or some other explanation underlying the lower results as compared to the higher results.
Why does Biden show seemingly two trend-lines with his polls?