QUESTION
Why does the cord visibility on the side of the belt differs from belt to belt?  
ANSWER
It is a feature of the belt production process. Individual belts are cut from a broad, continuous cylinder called a 'slab'.The cord is spiraled from one end of the slab to the other. After vulcanisation, the slab is cut into belts. The cord is visible on the side of the belt where the cuting crosses the cord.