Steverino's the kind of guy to take that sort of long-term overview, and if you think about it, did the updates applied to produce Mac OS 9 really warrant a full-integer version increment from Mac OS 8.6? I think not; jumping prematurely to v9.x was just part of Steve's masterplan for apparently progressive stages of evolution towards "X11 for the rest of us" (at least Mac OS X v10.x didn't have anything to do with those X10 cameras of accursed popup-ad fame -- but then again, now we have the iSight, hm...).
Interesting story about this one: Originally, OS 9 was supposed to be OS 8.7, but it was changed to 9 at the behest of Apple's marketing department. The most likely scenario is that 9.1 was originally going to be 9.0, since that's where all the truly new features were, but I digress. You can find evidence for the existence of 8.7 by replacing the 8.6 DVD player with a 9.0 DVD player (on an 8.6 machine of course), and try to play a DVD. The error that pops up will say something to the effect of: "This version of DVD player requires Mac OS 8.7 to run. Please use an earlier version of DVD Player."