I always viewed the difference as intent and not mutually exclusive. reason explains your thought process, actions, and events. Excuse are reasons presented in a manor meant to shift responsibility.
Sounds like a rigged game: if someone wants to feel like the blame is being shifted.
What good is blame anyway if you want to fix the problem in the first place? I'll never get it. It feels primally cathartic to blame a person, but it fixes the issues to find the reason.