It is true that many people working in fabric stores and other places that carry fabric, don't have a clue what fabric is all about, BUT please remember that there are those that really know their stuff. I worked for Joann's for about 5 years plus several other fabric store and also have been sewing since 6th grade, I know a lot about sewing but I don't claim to know everything. I would cringe when I saw someone cut 108" wide muslim instead of tearing it, or cut burlap instead of pulling a thread. Even people that have worked with fabric for years still make errors in their calculations --- I know I have. For all we know, the amount of fabric that she needed for her curtains did include her hem and header..... she may have been told by someone else other than the gal at Hobby Lobby what she needed. I guess what I'm saying is give the young sales gal a break --- it may have been her first day and was nervous as heck.
She will learn.