I think ckcowl's explanation of business vs. personal use applies to many who use the term studio. For others, it may be a difference in the location. If the quilting 'area' is in a separate building from the main house, it's more likely to be called a studio. Also if the quilting 'area' was specifically built, configured, or remodeled for its purpose then it's more likely to be called a studio. For others, it is probably mindset - a quilter has a room, an artist has a studio.