I'd suspect the reverse: perhaps higher temperate would generally be preferable but usually the breeding is in colder regions because the warmer ones have too m

I'd suspect the reverse: perhaps higher temperate would generally be preferable but usually the breeding is in colder regions because the warmer ones have too many predators, but those vents don't share that predator problem. Maybe the warmth outlier is too small to sustain the particular kind of warm-water predators they evade.