Could be a variety of reasons. Piston rings, valve seals, valve seats, carbon buildup and even the dreaded head gasket. A leakdown test would help pinpoint the problem. The numbers in themselves aren't that bad, but if you're buying the car it better be as good as advertised. More than likely the previous owner didn't want to spend the extra $$ on 91 octane gas and pinged the shit out of it. It should run fine, (it is a Honda) but for how long and how hard, I dunno.

Besides it's a GSR, you know you're gonna be revving the shit out of it every chance you get. Hell I do it in my cx, and c'mon, it's a cx, lol.