N is for Not My Kid

Ok, so being a parent I have to ask.  What's up with this thing where a kid does something mean, inappropriate, horrible etc and then when you talk to their parents it's all I can't see my kid doing something like that?  I mean my kids are a lot of things but perfect does not make the list. 
I am willing to own up to them doing things.  My daughter hauled off and slapped your kid?  While there's probably at least an 85% chance your kid provoked her in some way I have to admit, my daughter....she has a temper.  So she will be apologizing for her behavior.  And she'll be punished as well. 

Now, if you tell me the same thing about my son and yeah, I will have a lot more trouble believing you.  The kid is just not that violent, you would have to push him pretty far, and even then he's more prone to hit an object than another person. 

Either way they are pretty good kids about admitting that they did whatever it is that happened once asked their side of the story.  (even when it comes out not remotely in their favor) 
I just don't understand what makes someone think that saying to another parent "I'm sorry, he/she did 'xyz'.  It just doesn't seem like them." makes up for their behavior.  Whatever happened to making your kids own up to their own behavior?