I'm sure that the 2 quotes were not apples-to-apples. One or more of the quilting components - fabric/batting quality, pattern complexity, quilting quality etc. were different between the two.
At least you were wise enough to provide your quote it writing. I cringe every time I read a post where a custom quilt was made and then the customer didn't want to pay because it was too much. Putting everything in writing before you start is the way to go (along with a deposit!).