I found a couple things pretty interesting. First, in 3 out of the 4 studies, the bottom quartile's perceived ability was the least of all the quartiles. This means that though they thought their ability much better than it actually was, at least they recognized it wasn't as good as others.
But most interesting is that the perceived ability of those in the top quartile was below actual ability, showing that increased ability tends to make one more aware of one's limitations. This explains why we find the most tentative expressions coming from the most capable. Those with high ability know they're good, but they find it humbling as it apparently increases their awareness of how easy it is to go wrong.
The corollary, and your main point, I think, is that the most confident expressions come from the least capable, and this has unfortunate consequences when it comes up against human nature. People find confident expressions most appealing, and these studies tell us that people are most likely to follow the advice of the least capable.
This was illustrated on Seinfeld many times when one of the main characters, usually George or Kramer, would confidently give incredibly poor advice to someone that when followed invariably led to disaster.
--Percy