In the first example, the average-fitness person probably has a lot more variance and a lot less visibility on his physical performance than the Olympian. The Olympian presumably also has a selection of meta-skills surrounding his chosen discipline, and is capable of judging when he's off his game or when he falls short of his own standards.
In the second example, well...
I don't know if you've ever gotten into the timeless identi-discussion with someone who is literate, but refuses to learn how to adequately punctuate their sentences, because they don't see the difference. A lot of people decry poor standards of reading comprehension, and my pet hypothesis is that many readers don't actually parse and evaluate sentences, but just let their eyes suck in the words and have a good guess at what it's supposed to mean.
I've never explicitly stated that hypothesis, but now I have, it seems like a case of attribute substitution: i.e. "Parsing this sentence is hard, so I'll round it off to the nearest sentiment and assume that's what it's saying".
If you're interested in some actual research on that hypothesis, try Ferreira for a starting point. Any of the papers on her page with the phrase "good enough" in the title will be relevant.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.