When you swap primers, they all go off, but grouping opens & closes with testing.
When you adjust primer crush, which includes pocket depth & primer height, grouping opens & closes.
When you adjust striking, pin fall, diameter or mass, or spring, grouping opens & closes.
You may not notice this at 100yds, but if you're not testing it at distance, you're likely leaving something behind. You could suggest that so & so won once with mud packed under his primers. But that doesn't mean he couldn't have won, or done even better that weekend with more efforts.
I don't believe ignition affects grouping only, but SD as well. Another attribute to watch with testing.