Only way to get them on backwards is to put the driver's side on the pass side. If so, when you lide the seat adjuster, the seat spring will pull you backwards instead of forward.
Whe the covers get put on, they could be pulled more towards one side making the seat look like it is leaning. When someone is doing their seats, I tell them to look at the seat while standing in front of the car before putting all of the hog rings on. Mine looked perfect when I did mine on the ground. When I put them in, I noticed they were leaning towards one side. Funny that you didn't notice that when you saw my car. If you did your seats yourself, you would have noticed it right away.