I have absolutely no science to back this up, nor do I have any reason to believe this except that I've heard it on the boards before, but apparently as you lose you will see some "new" stretch marks appear, but they're really just ones you had before you lost. Something about your skin shifting and having more room to move and for some reason the color deepens on old ones you couldn't see before. I think if you think of your skin like a slinky it makes sense -- your skin is like an extended slinky until you lose weight, when it coils back together. And that re-compression of skin can result in color where you saw none before.
... Not sure if that helps, just know you're not alone in that problem.