It could be "lazy" eye, which is when one eye is not working the same way as the other. If you catch it early enough, I believe it can be fixed with a patch and/or drops.
My son has this, but since his eye was not turning out (as your daughter's seems to do), we did not discover it until his Kindergarten vision screening. His vision in one eye is perfect; the other eye is 150/20 . 200 is considered legally blind. If it's not taken care of (by glasses, for example), the "good" eye does all the work to compensate for the "bad" eye, and the bad eye will eventually stop working. My son will always have glasses now, as it is too late to correct it - but I'm just thankful we caught it now before it became even worse.
One other thing, I also had lazy eye when I was a baby, and my mom told me they were able to correct it with drops.