"Wobble" is indeed the correct term. The experimental observation is that the third position, in terms of the actual chemistry or steric criteria, is not as "stringent" as that for the first 2 positions. Because of the flexibility in the 3rd position, the genetic code is degenerate. Why is it this way? Various ideas about the value of a degenerate code have been posited, most having to do with ideas of mutation mechanisms.