The "2%" is a term used to describe someone who has broken through a social barrier or is allowed to break a rule. The term is derives from the fact that condoms are only effective 98% of the time (so the 2% of the time they are not effective) which means that if someone is described as the 2% they are being compared to the sperm that get through the condom (due to a split or tear) on a rare occasion. In other words the exception, not conforming to the general rules.
Jon: Hey you can't do that!
Dave: Yes I can mate. I'm the 2%!
Harry: *Gasp* Jack can't go in there!!
Sam: Haha, he must be the 2%