The frame clearances have changed over the production runs and it is possible you have one with the tighter clearance, what tyre are you measuring it off?
Roger
Sorry, I was in a hurry and I forgot to mention I’m running a Maxxis Creepy Crawler.
To bad they dont make a Maxxis Hookworm in 20x2.5. 
I had a friend buy a BMX tire and use it on his uni, he is all about trials, though wasnt big or anything, he said it was fine for jumping off stuff, but it wouldnt jump well.
doesnt really hold your weight to well, looks like you are way to heavy on it.
they can jump plenty high, you just cant prehop