Are Demon and Dart seats different?

I took a set of seat upholstery for a 71 Demon and a set of seats from a Dart to the upholstery shop, and they are saying that the actual seat cushion part of the rear seat is too large for the covers to fit.

Would the rear seat lower section for a 70 Swinger be larger than the lower back seat section in a 71 Demon? Or is there just something wrong with the seat cover lower section I got? Anyone really familiar with the differences in seats?

Thanks.