Well, but there is a difference between Mac OS X and classic Mac OS version numbering. Before OS X, x.0 and x.5 releases were the major releases, while x.0-x.1 and x.1-x.5 upates were mostly bug fix releases, maybe with some small new features.
Mac OS X 10.1 was much more than a bug fix release. It had lots of new features, it was much faster, actually the 10.0.4-10.1 update had more changes than the 8.6-9.0 update, still they only called it 10.1. However, with the new features came new bugs, and I'm sure that 10.1 had more bugs than 10.0.4.
Then they used +x.x.1 releases mainly as bug fix releases, just like e.g. the 8.0-8.1 update.