Or maybe Biden voters are not as enthusiastic as Sanders voters, so they don't want to tell exit poll people how they voted. I think you are grasping at straws here. Actual vote tallies are 100 times as credible as exit polls. There is so much transparency in reporting tallies that any number rigging would be obvious to too many people for it to be kept a secret. This is a very important factor in any conspiracy theory: How many people would it take to pull this off? Would that many people actually keep it a secret for x number of years?