Yes, context is important - and so far as excuses v. explanations are concerned, it is, surely, the context of relationship?

An explanation is given to someone with whom you are co-operating (in one sense or another) in order that they may understand why a certain event has taken place (or failed to!) and be able to participate fully in whatever action or inaction is then appropriate.

An excuse is offered to someone who has power to "punish" you, in order to get yourself off the hook!