Pipping wrong end but still hatching successfully

[LooseyGoosey]

Every batch of chicks I've had a handful pip entirely in the wrong spot. And they still hatch just fine. Last batch, I had one zip that NEVER pipped at all. I can confirm from hatch debris that one from last night's hatch never pipped the air cell, it just pipped the small end and the zipped.
How common is this, or how common for them to successfully hatch? And what cause my high rate of incorrect pipping? Is it caused by genetics? Incubation habits? What?

 

[Wyandottes7]

I've had several chicks pip on the wrong end of the egg. I've had to help all of them out, but once they're out, they act like normal, healthy chicks and grow just like the others. I'm not sure why some chicks pip on the wrong end-- I think that it may be a genetic problem, as most of my wrong-end pippers came from a specific hen.