Perhaps I made some assumptions. Lets see what I based them on. I learned to dive in warm water. I taught hundreds to dive in warm water. I taught a few to dive in cold water. I have more than 6,000 dives, including in Maine, New Zealand, and The West coast of OZ, places I consider cold water. I've run a liveaboard Dive boat for the past 15 years, and seen about 10,000 divers tromp across my decks. No, not all warm water divers (which I consider myself) are piss poor, and not all cold water divers are all that, but as a general rule, cold water divers have to learn to navigate due to usually poorer vis, have to learn what to do if their reg freezes, have to learn better buoyancy skills because of compression of their heavy wetsuit/drysuit.
Warm water diving is comparatively easier. When I was a resort instructor, I would take on a 3 day class without thinking about it. You can keep the students in the water longer, you can see the student when performing skills and you can see where you're going, negating the need for compass navigation. So, as a resort instructor I would make these 3 day wonders who wouldn't dive again for 6-9 months after certification, if then. Of course, by then their skills are rusty and they would need to follow the guide because they remembered to put the breathey thingey in their mouth, but weren't too sure what to do with the blow up thingy on the end of the hose.
By contrast, the few students I certified in 37 degree water in Maine kept diving. Dive instructors in the North aren't trying to push a class through so they can start the next class. They tend to take the time to run pool skills a little slower, because they know that when the pool is over, they will be getting in the damn quarry. The students know that they would survive cold water, and they would practice at the lake on weekends. The point is that they KEPT DIVING while they were home. That kept their skills up, that kept their minds sharp. Warm water divers tend to only dive when the water is warm, and for folks who live in the frozen north, that isn't as often as for those who will dive anywhere, any time, any temperature.