This is a joke but it ain't really a joke - if you're trying to figure somebody out you often can tell a lot from the jokes they chose to make and even how they tell the joke.
The most obvious and probably most familiar to most example is somebody who says something demeaning of somebody else present and then, when confronted with the insulting nature of what they just said, claims "it was just a joke".
Maybe (and this part is much more wild theory on my party) people feel they can get away with more in joke form and hence are more likely to share their true beliefs?!