Try and look at primers without the "magnum" name. For this discussion, let's call 'em HOT and regular. The reason I say this is that people confuse "magnum" with their cartridge. .... It's the load data and more specifically the powder that demands the HOT primer.
Those few sentences are about the best explanation ever given on these pages when trying to explain magnum primers. So many newcomers to the reloading scene tend to think that if their cartridge is labeled "Magnum" then so should their primer, and vice versa.