4 long shot/ SWAG's (silly wild a** guesses)
1) I've seen differences in neck tension cause that much variation. You might measure the neck thicknesses and see if they are all consistent (its a long shot). I've seen it when comparing different bushings or dies with the same load.
2) Over-Uniforming (enlarging) of the flash holes can also lead to velocity spreads. (another long shot)
3) How close was the chrono to your muzzle? I've had 'silly' readings when the chrono was too close (I think it was due to muzzle blast).
4) Had all the brass been fired the same number of times? Some new, some once fired?
Hope one of these helps,
If some is good and more is better, then too much is just right.
My mind is a raging torrent, flooded with rivulets of thought, cascading into a waterfall of creative alternatives