If it is the case that perception requires the use of specifying variables, then there should be no individual variation in what information variables people use. However, as we've already seen, such variation exists: the dynamics of a collision event produces multiple kinematic patterns in the optic array, and people judging the mass ratio of colliding balls use all of these, only one of which actually specifies which ball is heavier. Even with training, people do not always find the specifying variable.
This is an example of how the mapping between a property in the world (mass ratio) and the optic array can be one-to-many, with consequences for perception. Figure 1 on this post shows us that there is another mapping to investigate, namely the one from a perceptual array to the organism. Can this mapping also be one-to-many? Withagen & van Wermeskerken (2009) suggest that it can, and that again training does not necessarily help.