How is it any different from Android 4 Ice Cream Sandwich to Android 4.1 Jelly Bean to Android 4.4 KitKat, or Ubuntu 13.04 Raring Ringtail to Ubuntu 13.10 Saucy Salamader to Ubuntu 14.04 Trusty Tahr, or Mac OS X v10.7 Lion to Mac OS X v10.8 Mountain Lion to Mac OS X v10.9 Mavericks?
They're all the same OS (and the same OS release) with names tacked on to differentiate between minor releases.
Exactly. "Update 1" is, when spoken, shorter than "Service Pack 1" what would be the older convention for this kind of release of Windows. Moreover, the changes are supposedly bigger than what they'd allow in the old-style service packs, so it's fitting not to name it the service pack and still shorter.
They're all the same OS (and the same OS release) with names tacked on to differentiate between minor releases.