TNG used its fair share of reset buttons. Time Squared, Yesterday's Enterprise, Cause and Effect, All Good Things, Future Imperfect (Kinda), the warp bubble one (Kinda), Parallels, Tapestry.
Why are we constructing these roundabout explanations of the simple fact: The writer of a given episode only cares about that episode, and although the writer will pay attention to established precedents he's certainly not going to chuck his cool idea because it contradicts something.
DS9 was marginally better at continuity than the others, at least characters remembered when they were mad at each other.
But oh, what the heck. I can explain the difference in trills between TNG and DS9. There are two races of trills, one has spots and one has bumpy foreheads. That's not a stretch, humans look awfully different from each other. There are also two races of symbiants. One race believes in equality and sharing a life with their hosts, the other believes in fully taking over the host's mind.
Now, I can't explain why, in a world where earth was totally unified, the majority of the population is very caucasian. One would think there'd be a lot more Asians and Arabs walking around in the 24th century.