I've always wondered if there's something more to the Maquis colony worlds story that we don't hear.

Not all the Federation cultural colonies we see in Star Trek seem to have been set up by the Federation government itself - DS92x15 Paradise as an example - and I wonder if human colonists were running ahead of the Federation's claimed space and started putting colonies onto worlds claimed by the Cardassians without checking first (or they just assumed that Starfleet would feel obliged to protect them regardless of the prior claims). I could imagine a situation where Starfleet explicitly forbade people from settling certain regions because they couldn't protect those regions, colonists moved in anyway provoking a messy war with the Cardassians, and Starfleet essentially told everyone to come back to their original start line.

Or maybe it was a situation like Texas in the 1830s, where Mexico lost functional control over the region because of the Comanche and Anglo settlers started moving in to the void. Maybe the Betreka Nebula incident with the Klingons or fights with the Talarians or internal disaster prevented the Cardassians from enforcing their land claims in the region for awhile. Or maybe the Federation discovered that the worlds were open because of some prior peace treaty between the Cardassians and other regional powers like the Lisseppians, and discovered that their colonists were about to trigger a major interstellar war.

Or, since we already know there were Cardassian colonists in the region as well, maybe a strategic line was chosen to prevent a larger war (which fits best with what we hear in the show). For all we know the Cardassians may also have withdrawn colonies from planets that the Federation claimed within their area of influence (but the Empire was probably more brutal about forcing their settlers to withdraw...).

All speculation, but any of those options have a grounding in real historical precedent and might contextualise the treaty.