I've gotten the impression that not all successful challenges are "accepted," but some are paired with other open seeks. ... ?
I once played a fellow, and having won, got a rematch request which I accepted. After the second game, a waited a short time and hit the rematch button. The same opponent came up, but I soon got a chat message which said "You again?". Evidently, he had posted another new challenge, and my "rematch" request paired us without waiting for his consent.
I believe that there is a matching algorithm that works much as you've described Bur_Oak. As for why a 1500 rated player would match with a seek that had a rating floor of 1600, that does sound like a bug.
I thought there was a rule in place that prevented you from setting your rating floor to a value higher than your current rating. Could this be the issue?
As for why a 1500 rated player would match with a seek that had a rating floor of 1600...
It may be considered "close enough," -- not by the player having set the floor, but by the algorithm.
I have the min rating set to 1600, yet 1500 rated players accept my challenge.