This title is a somewhat rhetorical question - what I'm really asking is whether other WikiTree people have gotten into this enough that they can confirm or deny what I've been seeing in my own results.
AncestryDNA lists your DNA matches in decreasing order of centimorgans (cM), so right off I find the double categorization they do (Cousin levels & "Confidence Levels") to be a bit misleading. Every match in the "Extremely High", "Very High" and "High" confidence levels are at least 30cM, so as far as I know, they're all pretty certain to be a real match. The "Good" confidence levels go down to 16cM, so most - if not all - of THEM are real matches too.
AncestryDNA's various levels are basically fine, for the closer relatives. It can pick out a parent or full sibling "Immediate Family" with ease. I only have enough data from my own personal experience to discuss the "2nd Cousins" level and beyond, and that's where things get to be "off".
When I click on one of my matches in the "2nd Cousins" category, it tells me that the "Possible range: 2nd - 3rd cousins", but that's misleading. In my own case, I have two 1C1R and two 2C. When I look at my brother's results there is actually a 2C1R (who squeaks in at 213cM), but there's also a 1C (606cM). The rest of his are like mine. From what I'm seeing with my 3Cs, there's no way you're going to get a 3C in the 2C category.
So what "2nd Cousins" REALLY means is "2C or closer, with a small chance of a 2C1R sneaking in across the 200cM threshold". Almost all of the matches in this category will be 1C1R or 2C. But just as importantly - only about HALF of your 2C matches will actually be in this category - the others will be under "3rd Cousins".
Now, what does Blaine Bettinger say about 2nd cousins? He says you should see 46cM to 515cM, with an average of 233cM (or, if you look at his data, and use the center 90% of his results, it's 93cM to 390cM, with a median of 208cM). In my experience, they ran from 102cM to 326cM, with an average of 191cM. The thing is, he doesn't appear to throw away the endogamy-ridden data, that can be WAY off (so that's probably why he goes up to 515cM), and who knows if the fine folks reporting in really even know for sure what a 2C is. Anyway, his average is about 20% higher than what I'm seeing, and his max value seems downright out-of-control.
Similar deal, but I have much more data of my own. It tells me that the "Possible range: 3rd - 4th cousins", but in actuality it's the bottom half of my 2C matches, the top half of my 2C1R matches, and a relative handful of actual 3Cs It really means "3C or closer - as close as 2C." Out of the 70 3Cs I've identified, only 10 were in the "3rd Cousins" category (46 were under "4th Cousins", the rest were "Distant") There could easily be more in the "4th Cousins" category, and especially in "Distant", since I haven't figured out who all of those are yet.
NOTE: For 2C1R and closer, the values do not go down to essentially zero, and a side-effect of that is that you will ALWAYS get a match. For 3C, I'm getting matches to about 85% of the 3Cs that I know are out there (so the real number has to be less).
My highest 3C value is 127cM, and the average is 47cM. Undoubtedly, that average will drop as I find more 3Cs buried with my "4th Cousins" and "Distant" categories. Blaine's chart says it can go up to 217cM, and that the average is 74cM. As the relation goes further out, it seems reasonable that his endogamy data is throwing things off more and more.
Mostly, these range from a few 2C1R to some 4C1R, but a few as distant as 6C1R. But I haven't figured out who they all are - not by a long shot - so there might be a lot more at the more distant relation level that I haven't found.
As low as 3C appear there. I have as remote as an 8C, too, but that hasn't really been verified rigorously.
It seems to me that one could get somewhat the wrong idea about how you might be related to someone, based on the cM of a match, using the small amount of info we have had at hand. I'd be interested to hear what others have seen for themselves!