Some of these are pretty hilarious stories. A lot of my thinking about this was influenced by Allen Fox, who advises you to keep your excuses to yourself.
Tennis is a pretty rough game, mentally, and it feels bad to lose. Most of the excuses people make are an attempt to minimize the relevance of a loss (or maximize the relevance of a win). A loss is a loss, but you can feel better or worse about a loss by attributing it to some factors rather than others.
I realized I do this too. I lost a pretty grueling semifinal in a tournament 7-5, 6-4. I led 4-1 in the first, and trailed by a break in the second, evened it up by pursuing an aggressive net game, and was broken again.
I've learned to never share my post-match analysis witho my opponent, but the story I told myself (and my tennis playing family members), was that I went ahead, but didn't have the fitness to continue. As I started to wilt, I switched to a serve and volley, net rushing game, which went well until my first serve percentage dropped.
Now, maybe that's true. My opponent was fitter, and younger, and I do need to improve my fitness. But it's still a version of events that allows me to see myself as the more skilled player. You see, I won when it was about good strokes and tactics, I just lost to a younger jackrabbit on a hot day. The second break in the second set was all about me and my first serve percentage - it had nothing to do with my opponent's good play, right?

I was minimizing the relevance of my loss, convincing myself that I was actually the better player, trying to convince myself that the sections where I was winning were more reflective of the real match.
My opponent did share his post-mortem with me. He said that he had been "nervous" in the beginning, but then he settled in and started hitting his strokes properly and then turned it around. He was "thrown off" by my aggressive net play, but then he settled and stared hitting good passing shots. He was maximizing the relevance of his win - disregarding the sections where I had led as "settling down" and convincing himself that the parts where he won were more reflective of the "true" contest.
Was he right? Well sure, why not? So was I, it's just completely irrelevant and nobody wants to hear it. You just embarrass yourself with these post-mortems. Allen Fox points out that our excuses are *usually* true. Ok, sure, you occasionally you get the total yahoo who utterly mischaracterizes the match - a low 4.5 hacker who was blown out by a 5.0 with a very pretty game actually (and hilariously) said he "couldn't handle his opponent's junky strokes". But usually, people are pretty smart and good at deluding themselves, so they pick reasons that have a grain of truth.
I still keep my mouth shut, but it's interesting to hear the reasons people give. And it can be a good strategic exercise to think of how your opponent would describe the match.