If only I knew. The logic was introduced by @parkr in 1f23bc4dc01a7ed88b75ae0204a1ed06bf2cd32d and 0e82b4eb2f49873be0e84e8deaf10eaee813733b - I just moved it away from its [current location](https://github.com/mojombo/jekyll/blob/master/lib/jekyll/page.rb#L66).