Because unlike the rest of us, she's had a normal childhood; her father isn't around home a lot, either, but she didn't get brought to the station, so she's been insulated from it. She's never been given a reason to think this way, and we all took it for granted that she'd somehow know on instinct alone what we know because we were raised in it.

She knows now, what that kind of thing means, and that she did risk your life there - she wants to be a cop, so I have an easy way to volunteer to teach her survival skills without flagging it as suspicious.