Any close race or race with many candidates is likely to have multiple ballots.
Read to the end of this article and also take note of the brewing controversy.
It is notable that while the MSM polls have Biden out front by a wide margin, it is Bernie (like Trump) who draws the enthusiastic, large crowds while Biden can’t fill a high school gym.
Also, they are hiding Biden just like they did HRC (because he hurts himself with public appearances) which is going to exacerbate the perception of a fixed primary to Bernie supporters who see their guy everywhere drawing big crowds while Biden does nothing.
Link: https://m.washingtontimes.com/news/2019/may/29/bernie-sanders-supporters-distrust-joe-biden-poll-/