Apple has fewer advisories because it's their standard operating procedure to sit on security bugs for several months and then patch them all at once, even if their contemporaries are patching them as they appear.

If you look at the numbers for OS X as opposed to Windows XP, OS X has 1,544 vulnerabilities in 153 advisories (~10.1 vulns/advisory) and Windows has 472 vulnerabilities in 358 advisories (~1.31 vulns/advisory).

Unless you have a good reason to believe that bugs in Windows are nearly eight times "more unique" than bugs in OS X, please don't compare advisories.