At some point the popular wire insulation changed from a kind of super-flexible silicone-like plastic to a resin-based PVC-free version. Partly for environmental reasons, but it had the side-effect of making wires a little stiffer and slicker. It's also why a couple generations of Lightning cables were known for splitting at the bend point.

I'm not sure if this 100% explains what you're seeing, but it's probably a factor.