My seat covers from World Upholstery looked like that with the upper part of the back too long. It looks like it should be wrapped around the cross bar that runs horizontal across the upper back, but it really should be wrapped around the upper rounded part of the seat back structure. It will basically wrap around about 1" of the back perimeter. Does that make sense? Then the seat back cover will slip underneath the perimeter.
Bill / 1977 Alfa Romeo 4C2000 / 2012 BMW S1000RR / 1975 BMW R90/6