It may be attached to your level but the farther I got in the story line the more there were. Survival took me a little ofver 24 hours to finish the main quests. And more and more ledgendary mobs would show up. It was a pain at lvl 17. Started on normal just to explore. There is a big differance. have only seen 6 or so ledendary mobs and lvl 22 now. But not that far into the story also.

http://fallout.wikia.com/wiki/Fallout_4_creatures(admittedly I haven't checked any game files, but by experience it does seem to be true. There also seem sto be a gradual phasing in of legendary enemies up to lvl 20 or something, but I've found plenty before that. That might just be biased false-confirmation guesswork tbh.)