Machines that don't have a built in walking foot or an attached walking foot will slightly ease the bottom fabric to the top fabric. That is why when you sew on borders you want to sew with the quilt top on the bottom and the border fabric on the top. If the border fabric is eased even slightly your borders will start to get ruffly.
Buying a Pfaff with the built in even feed system is one of the very best things I did to improve my accuracy.