It's accepted, I believe, by most that bullets stabilize more the further they travel from the muzzle, up until the point of going sub-sonic, of course. So, those bullets that shoot better at 550 than 100 may be "going to sleep"; or stabilizing and getting past any yaw in the spin.
Also, the shooter may just be better at that distance than 100 yards. I know that I definitely force it a bit at 100 when testing loads, and have the tendency to shoot poorly.
That's just my stab at it, anyways.
