Apple has enjoyed the fact less companies target OSX and, assuming the same competent to incompetent developer ratio for both platforms (something risky in itself, because the barriers of entry for Windows are much lower), it would be no surprise if there were less buggy programs for OSX.
Also, OSX is a bit older than the current Windows platforms. Don't forget OSX is a direct evolution of the NeXT operating system and that makes if a good four years older than Windows NT. It also builds on both the Mach microkernel and the BSD more conventional kernel, so, its plumbing is a tad more mature too.