If I understand your question right, you're saying that the primer has a line on either side of the detent where the firing pin struck the primer. If that be the case, then I'd say there is nothing wrong with the pistol. Out of the 20 some glocks that I've looked at their spent brass from, they all do that...it has to do with the way the firing pin is shaped. I'd say don't worry about it, and enjoy shooting it.
~~~Amos
P.S. If you want to see what I mean, the next time you disassemble the pistol to clean it, push the little round button on the bottom of the slide behind the extractor while keeping the front pointed down. Then examine the shape of the firing pin. It's not round like some pistols, in that it is more of a flat shape with a point on it. HTH