I've had that happen with some 5.56 cases, too. It's as if the primer crimp was less pronounced than it is on cases using large primers. Part of it may be the narrower small primers are more rigid and do a better job of helping to push the crimp out during decapping than the large ones do. The primers that are able to seat this way usually feel like they are going in fairly stiffly, though, and not with the same effort in every case. So, I would expect it's going to be conducive to irregular ignition and wider velocity ES.