Bed Rail Measurement

[hijacker]

I’m planning to get a rolling tonneau cover for my 2019 GMC Sierra 1500 crew cab. I’m not sure why, but I measured the distance between the top of the bed rails at the cab and at the tailgate, and the distance at the tailgate is over one inch less than at the cab. With all of the trucks I’ve owned, I’ve  never had occasion to take this measurement and so I don’t know if this is fairly common. I’m afraid a rolling tonneau cover would start binding as it is rolled toward the tailgate. Has anyone else had any experience in this area? Thank you.