I'm not sure about the YCVC notes in particular, but if I had to guess, they are probably uncapped with a so-called "Most Favored Nation" clause that gives the YCVC safe-holders the benefit of the founders' most favorable (to the investor) terms.

The example of the safe we posted is specifically one with a valuation cap and no discount. But there are different options available, all available on YC's site: http://ycombinator.com/safe/

The YCVC safe is uncapped.

Optionally