ZydoSEO - 2:29 am on May 24, 2013 (gmt 0)
They could have differentiated this version because the algorithm itself had a major rewrite. Cutts did say that that the previous version "looked" at the home page (while it could still have "affected" the entire site) and that the new version "looks" at inner pages. That would likely require a major overhaul resulting in a major release rather than a minor update or data refresh.
I agree that there were likely not just algorithmic changes, but also infrastructural changes to allow them to roll out updates in real-time and more frequently without requiring days to roll out. This update rolled out in a matter of a few hours. If I recall correctly, previous updates rolled out over several days.
Search Metrics did post some interesting data on their blog about which sites they are seeing as most affected by the update... #*$! sites, gaming sites, and even some major brands like Salvation Army and Dish.