You see that a lot with federal gmm ammo in 223. I've seen it in other situations but its not that common.
The 2 most likely causes, soft brass or over pressure load. Other than the chance it could be dimensional tolerances...
6.8 might be weird because I've seen that same thing with RP brass a fair amount and quit using it in 6.8.
May the road rise to meet you, May the wind be always at your back, May the sun shine warm upon your face, The rains fall soft upon your fields and, Until we meet again, May God hold you in the palm of His hand.