I have heard -- don't know if it is true -- that a manufacturer will send what are called "seconds" to the non-quilt shops, i.e. WalMart, Hancocks, etc.
These goods are, as has been said, on a less expensive gray good. OR, they may have not taken the dye correctly, or the dye is splotchy, or something. It pays to open the bolt and look closely at the length you are buying to be sure it is not going to cause a problem when you cut it.
AND I am with everyone else -- buy the best you can afford. But DON'T allow the price to stop your creativity. Do your best, enjoy the process, and let the devil take the hindmost!!! :twisted: