It doesn't increase the amount of tar, it increases the ration of tar to THC, in other words, the level of THC is decreased. I know THC isn't water soluable but its what happens. THere are studies all over the web about it, I think some one had posted a link to one recently, try the vaporizer threads. Apparently the most efficient/healthiest way to toke, next to a vape, is a joint. I don't see why, maybe the weed filters the tar well at the begining, but then your'e just smoking it all eventually anyways. I think it has something to do with the way they're testing it, I'd like to see the tar/THC of a nearly roached out J.
Peace