The simple fact is that stockinette curls. Non curling borders can help some, but they aren't perfect either. They sometimes fold over into the stockinette. Here's a few links that may help along with other suggestions you get.
These are all about combating the curl whether it's a button band or whatever so there are various suggestions and more links at the bottom of the article.
http://techknitting.blogspot.com/200...zag-bands.html