I like Venomballistics answer. just adding two cents, if you don't have a revolver I would say that your cases were slightly different lengths at firing. A thou or two can change the look of the primer strike. I had a similar problem in .45 auto about 20 years ago when I started reloading. One of the guys at the range told me to make sure my primer pockets were clean and the cases were the same length. I became kind of anal for a few years about keeping cases clean and making sure I trimmed each lot after every firing so every case was the same length. I used the lee case trimmers and the lyman primer pocket cleaners. I cleaned the primer pockets and trimmed the cases watching TV in the evening. Never had the problem again. FYI: I was reloading pistol ammo on a single stage press so I always took my time with every cartridge.