Could just be the discount for the ease of throwing the boxed bolt to the shipper from the online warehouse.
Almost no labor or tracking or inventory or shipping costs for Joann's.
I'm not saying each bolt costs $150 to get to the store and cut by staff, but it's not zero either.
Anyway, I think the $6/yard is a great price, and I can buy as much (or as little) as I want/need.