MonoKote does shrink more in the direction of the length of the roll. This is true of all thermoplastics. The film is produced most likely from one of two methods, blown film, or cast film. In either case, the process orients the molecules in a fashion that yields slightly more shrinkage in the length direction.
The downstream lamination and adhesive processes will also contribute to this effect.
Last but not least, the pigment used to produce each color reacts slightly different, causing yet another possible shrinkage change.
Although the shrinkage is measured in thousands of an inch (.001") it will add up when covering a longer wing panel. I agree with the suggestion to keep the length of the roll consistent with the length of the wing panel.