Molecular identity is a malleable thing-particularly in the case of nucleotides.
While we tend to draw the bases as static molecules with only one form, in water solutions this is simply not the case. Indeed, at the time when Watson and Crick were guessing at the structure of DNA, the 'evil twin' structures of the bases were thought to predominate. This, of course, threw a major wrench into the model building, and it was only when this error was realized that the structure came forth. The different structures arise from the alteration of single and double bonds in the ring systems of purines and pyrimidines. The key fact is that tautomerization alters the basepairing properties of a nucleotide.