What do photogs really mean when they tell a model she has so much potential?

I've just started modeling with a particular photog. I've done 3 shoots with him so far and I am now part of his main group of regular models In spite of this I know I have many flaws. I tend to frown slightly at times (which once caused a huge loss in shots) can look flat, too posed, not sensual enough. While he often says glowing things about the other models up on his site, he hasn't said a great deal about me apart from token politeness like 'she's great, a work in progress'. He tells me he loves shooting me, and of course offers constructive criticism. I know that the other models are more polished, and I wonder why he keeps shooting me when he can see so many flaws. He's talked about how all the other models are so great. I guess what I'm really asking is why photogs may continue shooting and working on a model when there are other more polished ones out there. When do you decide that someone has 'potential' and when they have nothing? How do you know when to keep working on them