Ceiling fan mounting bracket issue
That is a joist centered fan rated ceiling box. I installed one of these during my remodel 2 years ago. It is a standard mount, so I'm not sure what you mean by the mounting doesn't fit. Show picture of the fan mounting plate. The box should have 2 holes on each side. Looks like one is partially covered by ceiling texture. Your mounting bracket should have elongated holes also, to help center the bracket over the holes.