TinysTeeShot: Also, it don't matter if you play 9 or 18 holes, if you shoot a 7 under it will drop your avg the same amount, for a 9 or 18 hole round.
It certainly does matter if it's a a 9 or 18 hole round. I believe that for all courses, 7 under for 9 holes is going to be (at most) a 29. That works out to a 58 average. But 7 under for 18 holes will be in the 60s and could be as high as 65 (if I recall par for all of the courses correctly).
In other words, the 7 under for 9 holes can certainly drop your average more. In fact, it's possible for a 7 under for 9 holes to drop your average, while a 7 under for 18 holes having no effect. That's the advantage of only playing 9-hole rounds rather than 18-hole rounds. An excellent front 9 can't be "ruined" by an average back 9.
As alosso mentions above, the average is not calculated by + par; it's based on total (or raw) score.