How did the Academy manage to pick a group of nominees that are so diverse that even Roger Ebert says it is a great selection (in his column here?)
One reason is the proportional voting system that members of the Academy use to pick the nominees. Voters get a first choice, a second choice, a third choice and so on. Roughly speaking, if any candidate gets 20% of the first-choice votes (and that's not a lot), that candidate gets one of the five nominations. So a quirky movie that's beloved by 1 out of every 5 members of the Academy can get nominated.
Here's a briefing I did a couple of years ago on the Oscar voting system for the entertainment press (when I lived in Los Angeles).
And here is a story in www.slate.com on the same (I'm quoted! Thanks, Timothy Noah!).
(The only post script -- my sister's short animated film The Toll Collector was up for a nomination, but didn't get one of the three slots. Perhaps if there were five slots, she would have been nominated. Academy, how could you!!)