The extra width is needed for the machine not to bump into the side clamps. If they load it with the selvages (ie - you leave it as is and do not cut off the selvages) it should be fine. If you are concerned I would call the company and and have that conversation. I have at time had to use a narrower quilt back and if it is too narrow for my likes, I baste a strip of muslin to the sides so I can clamp it but imho you should be good with that fabric.