What is the cause of different length legs? (1 leg shorter then the other) and the average percentage of people affected

A lot of reasons: To have one limp shorter than the others 1-sclosis(curvature in back). 2-pelivc problems 3-one bone in the leg shorter (femur, tibia). 4- there are rear causes also. I do not know the percentage of the people how have that.

Not uncommon: First of all, most people are not perfectly symmetrical. I find that 60%+ people have some leg length discrepancy. Most can't tell when they are only 1/4" off. There are many reasons for it, including: curvature of the spine, temporary growth center interruption of the tibia or fibula from injury when young (from jumping out of a tree to an auto injury causing mild compression of growth plate)