When 4/6 was a kid it learned what forms (or costumes) it could “wear” by observing other fraction's ID cards. (Or in the movie how many cakes they would eat. 4/6 always ate 2/3 of any cake. Yum.) 4/6's early discovery was that the costumes it could wear had the same size shaded part.
  
Here's a “lineup” of 3 of the ID forms that 2/3 can wear. Notice the equal shaded parts. This means that all of 2/3 forms (or costumes) are equivalent.