The difference in the looks is determined by whether the top layer is unidirectional fibers (common to the pre 2005 Treks- aka splotchy) or if the top layer is of a woven material- the checkerboard. Its not true that a woven top layer is "cosmetic". The woven material provides a more isotropic final layer, providing strength in mutliple directions. This is often doen to better absorb imapct.

It all just depends on what the engineers were trying to achieve in terms of fiber orientation for the outside layer. There are tradeoffs for every combination of fiber orientations. The outside layer may or may not reflect what is beneath it.