Windows 8 and 8.1 are receiving their own updates I think. Whenever I get an update in Windows Update, it's usually for just Windows 8.1.
Take a look at my update history:
Note that before I upgraded to 8.1, I had hundreds of updates previously installed for 8, but it clears the update history when you upgrade and it also changes the OS installation date to when the 8.1 update was applied. I originally installed this copy of 8 on my PC on April 27th I think, but now it reckons that Windows was installed on October 19th, which is when I upgraded to 8.1.