I don't know, she looks beautiful either way. She may have had a more balanced richer diet with her previous owner but short of the bigger belly, she looks pretty healthy with you. I love buckskins of every color from the creamy buttermilk to the dark smooty. You have a very gorgeous girl there. :)
I just took a second look though and I think it might be more than nutritional or environmental issues. It looks like it is just her genetics that lightened her up, IMHO. If you look at the first pictures, she didn't have any guard hairs on her mane and now she has lots. This may be her real color. I don't think that sun bleaching could do that.