I spent a couple of summers as a lifeguard at a college swimming pool, and I have seen this first-hand. As a lifeguard, I was well trained in how to rescue but not particularly well trained in how to recognize a drowning victim. There was a time I watched a boy bobbing up and down in the water, looking upward, mouth open, but not appearing to gasp for breath, the whole time wondering if he was in trouble. Luckily he…
My guess is that the reason was that she was not drowning yet. She probably would have if you hadn't saved her, but in her case she had a few pre-drowning moments in which to react. The difference would be that she consciously knew that she was screwed as soon as she was in the water, since she didn't know how to swim.