Sorry to disagree: A reason - even if given after the fact - differs from an explanation in that the latter disowns intent and responsibility for the causal chain involved.

An explanation does not have to disown intent or responsibility.

"Why were you late getting home?"
"Well, I thought I could stop at the mall to pick up something, but it took longer than I thought."

In this example, responsibility is being accepted but the rationale for the decision is being given. Excuse is the word I would use to descibe an attempt to weasel out of something.