Magnum primers are used for slower, hard to ignite powders, aka "Magnum powders". The data in the manuals are just reports of what the lab techs used and the results, not hard and fast formula. Also components vary from lot to lot and one of the reasons reloading manuals differ in their data. Your load sounds fine.
I have used mostly standard primers in my Magnums with the exception of when I'm using H110/W296. I've also read some use magnum primers with 4277, but I haven't tried that powder...
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