I'll give the following example which I often find myself having to say; "I'm unable to go shopping because my stress and anxiety levels are too high at the moment, although I would like to go it's not going to be practical and I'll need to return shortly afterwards - or even before getting there - which is a wasted car journey".
To me that's an explanation, and I'll always give one where I can. The same applies to the aforementioned balance problems described by franknfurter <- that's not an excuse, it's a perfectly valid explanation.