ur right they are different:
joke is something u tell for the amusement of others. the point is that the other person doesnt guess at anything, they just listen to the joke and say "lol" or "do i even know you?"
riddle is a usually funny situation where the person telling it is asking the listeners for what they think is the answer, which usually ends up being not what they expected it to be.
anyhow i think y! was trying to find main categories so i dont think its that big of a deal although i see ur point.
