There is a difference, but ya'll ain't even close.
The 7mm Express has more powder in the factory ammunition than the .280 ammo, of the same era. Not sure what else is different, unless you know of some other subtle factory loading issue. In all seriousness and reality -- I've owned my current .280 and my RCBS 7mm Express dies since 1985, so I do have a small clue about their relationship...
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Dennis