Unless she's a girl, then it's closer to 4,000

This is from Nancy Lublin's talk of TED.

so confused with its meaning.

I understand in context : If she is a girl, then it is closer to 4,000

But grammartically this shoud be underdtood this way : It she is not a girl, then it is closer to 4.000.

Please help me......

I attach part of the original talk is:

But the parents in the room know that texting is actually the best way to communicate with your kids. It might be the only way to communicate with your kids. (Laughter) The average teenager sends 3,339 text messages a month, unless she's a girl, then it's closer to 4,000. And the secret is she opens every single one. Texting has a 100 percent open rate. Now the parents are really alarmed. It's a 100 percent open rate even if she doesn't respond to you when you ask her when she's coming home for dinner. I promise she read that text. And this isn't some suburban iPhone-using teen phenomenon. Texting actually overindexes for minority and urban youth.