Unweighted is Ancestry's term for the match lengths between individuals before they apply their Timber algorithm. So although Ancestry claim that Timber generally makes their results more accurate, which most of the time it probably does, the unweighted results should align more closely with match lengths when comparing results across tests taken with different providers, and probably to some extent when using tools such as DNA painter; although the trouble with the latter is that neither we, nor the site itself have any idea of how many results input by users are using Ancestry weighted match lengths, Ancestry unweighted lengths, or those obtained through non-Ancestry tests.
Most of the time, the differences are minimal, but they must skew the results to some degree.
With regard to the figures quoted by ozdelver though, with the exception of the 9cM match, the lengths are significant enough to be considered genuine matches with some confidence, although 16 cM is on the low side of that assertion. Nevertheless they are all fairly low, and the differences are not so significant that I would worry too much about them. They might just be glitches in the test results for whatever reason.
Personally, I have to say that unless I had a good idea who those matches were and how they might fit into my tree, I wouldn't be bothering to investigate them with those match lengths, unless I thought they were likely to corroborate or prove a particular and significant line in my tree that is otherwise unproven or doubtful on the evidence I had so far, i.e. I really needed them to support my assumptions.
I did notice a few months ago that I had a match with a distant maternal cousin on Ancestry, which showed a shared match between us to one of my paternal matches. It was the only paternal match shown amongst ten other shared matches, all of which are on my maternal line and directly related to the match herself. I did briefly investigate it at the time and discounted it as my tree is already fairly widely researched and there were no geographic or other obvious possible links within a reasonable time frame. I checked again after the recent Sideview update, and the paternal match is no longer showing as a shared match between me and the other individual, so I'm pretty confident that it was just "noise" in Ancestry's matching algorithms.
