Most stringers do not do a good job of stringing the mains. To compound this problem, the recommended 10% higher tension for the crosses over the mains makes the problem worse, because improper stringing of the mains causes a significant loss of tension. The problem is improper loading of the racquet, as distinct from improper clamping of the racquet. The worse affected are the two middle strings. To properly load a racquet at the start of stringing, it is important that each of the two middle mains be pulled and tensioned separately at the throat end. In addition every main string should be pulled and tensioned individually, alternating first on one side then on the other side. If you do this, you will find the end product to be better and you end up with a more taut stringbed, with a significant reduction in your middle main strings moving during play.
I agree. Many stringers tend to speed up the process for massive production. Therefore, even using cranking machine, many tend to pull tension only on every other string (2 together) to save some time. With my drop weight machine, pulling 2 strings at once might end up wasting even more time, therefore, I always settle with tension each piece individually.