A long gone shooter I had more than 100% confidence in spoke about barrel node vibration. His claim was that it was most accurate in 800 fps increments with the 1/2 nose being almost as accurate.
It follows that the most accurate loads would be at 800, 1600, 2400 and 3200 fps and near them 1200, 2000, and 2800 fps.
Recognize anything familiar about the loaded velocities for many hunting loads? This would cause that loads outside these velocities with closer extreme spreads to have a larger group than those with a larger spread at those velocities. That was what I could gather from his thoughts which were at a far advanced level than mine.

Also as another late friend asked " When did a group become less than 10 shots?"

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Front sight and squeeze