Why did my rep go down? Is it because I recieved upvotes for answers that were later deleted (say, because the question I answered was deleted)? Is this the same thing that would happen during a site-wide rep recalc? I'm just curious (and tired, so I probably missed the entry in the manual about this feature).
Yes, it is the same as what would happen during a site-wide rep recalculation.
Your reputation would go down for the reasons detailed in the blog
Questions and answers sometimes get deleted; votes on deleted posts do not count toward your reputation.
Users sometimes get deleted. When a user is deleted, all their votes are deleted, too. Some of those votes may have been on your posts.
Questions get migrated from one site to another, which means any votes on that question and all of its answers are effectively deleted from the source site (but will now count toward your rep on the destination site, assuming you have associated accounts between sites).
Posts can automatically change to community wiki mode, which means any new votes added after the community wiki date no longer count towards your reputation score. This doesn’t actually cause a problem with chronology of votes, but I am mentioning it here for completeness and to emphasize that votes on community wiki posts don’t generate reputation for anyone.