Each part determines the stats not only for its main type, but they will also have gen sub routines that will not change their appearance, but they will change the stats they provide. For example, you can pull an Exobot with a Groundsmasher left arm, but it may also have Stronghold and Deathbringer gens, which will ultimately decide the final stats they apply to your Exobot, so even if two pieces look the same, they are not necessarily identical.