I have had a similar experience recently, and have decided that if I need a specific measurement i.e. 17", I will buy yardage, not FQ's. I have purchased them for smaller pieces and they measure anywhere from 17" to 19" x half the width of the fabric before straightening. I did complain to the (rather well known online shop) and was told that FQ's measurements were approximately 18" x 22" and that was that. That's when I decided to purchase yardage instead of FQ. That gives me the measurements that I will need, and hopefully a FQ that actually does measure 18" x the width of the fabric for future use.
I don't prewash, and hand dyed fabric probably is pre shrunk d/t the dyeing process, so even if it started out as a correct 18 x 22 before it was dyed, it might not be after.