You can try to explain anything in the show. Maybe the walkers heard the gunfire, but then got distracted and started attacking the group again? Maybe this...maybe that, but then you have this, and then that....
All speculation. My point is that the show itself needs to show something happen if the same thing has happened before. Be consistent. If gunfire (one bullet that hit Carl) distracts the zombies, then multiple gunfire needs to have the same affect, and they should have showed it. Instead, they showed no effect and the zombies kept coming after the group.
I'm a huge fan of the show, and will never stop watching. But while this episode was entertaining, there were many scratch your head moments, most of which has been stated here. Sure, you can try to dismiss them and make a cover story of what really happened, but most of see the errors and just shrug our heads.