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My uneducated opinion too, but I get it with every coffee and basket I've tried. I've always put it down to tending to have a little more headspace. In which case you might find certain coffees do it because they are more dense so they leave more headroom. I remember trying darker low elevation beans once which barely fit in the basket. True story.
Apparently a little variation in headroom has no impact so I don't worry about it.
In my uneducated opinion that’s a coffee issue. I get that with certain roasts and not others with all other factors being equal