The way danger is calculated basically takes into account how hard you and your foes are to kill - it calculates how much damage you're likely to dish out (ie. your accuracy, damage, speed) and how much punishment you can take (ie. dodging, blocking, HP), then does the same for monsters, then compares this to get relative danger value for you and monsters you can see in wizmode spoilers. If you look at the danger value, then equip better gear and get more stats, then look at the values again, you'll notice that danger of monsters have dropped, as they will have harder time killing you. When spawning monsters, the game looks at normal danger of the floor it tries to spawn monsters on - and because danger of monsters drops as you get stronger, it can spawn stronger enemies in lower danger areas.
Basically, because you have more HP, you can take more damage before dying - so the game spawns monsters that can dish out more damage to compensate.
I understand that it's rather weird how more health means you're more likely to die, but changing the way danger works without completely upsetting game balance would probably be a nightmare...
Another thing, though, is that some very dangerous monsters use HP thresholds to limit their spawn only after you have that much HP. This can cause problems if you gain large amount of HP without the gear to go with it, and this might be more easy to rectify. Maybe we could change HP thresholds to some gameplay milestones, adding flags like "do not spawn this monster before the player kills Jenny", "do not spawn this monster before the player descends at least to GC4 (but spawn it anywhere afterwards)".