Often it has to do with performance at each length, and how long the ski feels. They will try to build a ski with enough different lengths to suit everyone. The most key factor in the equation is most likely the sidecut and the resulting turning radius of the ski. Sometimes they may do it for marketing purposes as well. If a very short length in a GS ski doesnt sell one season, they may increase the length a slight bit to see if more people buy it. Personally i like to see when ski companies build skis in 5cm increments. I think that it is the best way to get the exact ski that suits your needs.