Ok
Nothing abut the speed of light.... But if you bend the cables or if the connector is not clean or if it's not 100 procent connected you'll get a so called thing named "jitter" and that's what I think it is... And too "not expensive cable " will cause this problem.
I've been there.
So...,, this have nothing about speed to do.
But please learn how it works in the digital world before you find it strange.... Pace