My first guess would be backwards compatibility - the "?=" is easier to implement with old tooling, so maybe they changed their URL scheme at some point. But in

My first guess would be backwards compatibility - the "?=" is easier to implement with old tooling, so maybe they changed their URL scheme at some point. But in that case they should have done one/both of: 1) Only generate the ugly old-style URLs for the old posts. 2) Make the old-style URLs into redirects to the new ones, rather than duplicates. This way must be hell on the search engines.