We noticed the same thing when loading identical loads in .257 Rbts of Speer 100g spitzers and 100g Nosler Ballistic Tips. The rifle was regulated for the Speers, and we were just checking to see if there was a group size change at 100 yds. Not much of one, but the groups moved up and to the right by about 2.5"!!! The change in height absolutely could not be accounted for by the better ballistic coefficient. .25" I could understand, but not 1.5" or better! Then there was that huge jump to the right. Harmonics, I would guess. This surprised me, because this barrel's contour is described as "semi-varminter," and is really pretty heavy. I wondered at the time if the difference in ogive, etc might cause the bullet to engage the rifling different, causing different air friction and Cornelius effect and... and...
Yep, let's call it barrel harmonics.
Will you, too, be one who stands in the gap?