Why is Pavarotti's interpretation of the Bellini songs different from Rolando Villazon's?

Why not? Even though both singers are operatic tenors singing classical music that is meant to be sung largely AS WRITTEN, there is still a great deal of leeway given to the interpretation of music as well as any actual stage role the artist is chosen to play. Intelligent and sensitive interpretation of music is the musician's contribution to the art of making music. I encourage you to look up even more videos of different singers performing the same songs and/or arias and see if you can figure out what is different/same about the singer's choices and how those choices affects the music and what you yourself personally like or dislike about those artistic differences.