1) Great team/culture
2) Everyone put forth the effort (scheduled crunch)
3) Some amazing techwork or artwork or designwork etc...
1) Didn't anticipate x/y/z
2) Burnout (remember that crunch)?
3) Bad/lazy scheduling and/or capacity planning
Kudos to the people who did this at MS but you're looking at tainted data. Most game developers know what really went wrong and do not publicly broadcast it in fear of losing future contracts or publisher trust. In the end the postmortem becomes a soundboard for 'shoutouts' and cheer-leading instead of that raw, unbiased feedback.
I've been to all hands post-mortems that were like that and it can get really really ugly.
Said director of engineering literally watched us blow milestone after milestone yet somehow the knowledge we were going to miss our commits (and not by a little) never propagated upwards to the ceo. Either that or the ceo didn't want to listen; it could be any combination of those two.
Either way, the engineers were pissed.
Incredibly poor milestone management and a failure to triage didn't end up in our fucking learnings document somehow.