based on her policies, warren is in fact in the same lane as buttigieg and biden. she's only mildly to their left.

sanders is in his own lane, and second choices are largely irrelevant. so, you would expect biden would come out second, as a balance of probability, due to name recognition.

stated differently: if sanders supporters truly have no second choice, you would expect the surveys to list biden second most often, based on the fact that he's winning. 

so, the data is not confusing, you just need to undo the propaganda to understand it.