Generally, I think the composing muse inspires composers more on some days than others. That makes for some variability in output. But then again, why should we be surprised? How often have you liked reading a certain author only to be greatly disappointed by one of his other books? Or have you ever been in an art museum and been enthralled by a painting by your favorite artist only to see another work of his that left you cold?
Another factor is that as a composer, artist, or author ages, he or she might enter different stages of output, such that what used to be considered "characteristic" is no longer so. Look at the difference between Liszt's, Beethoven's, or Scriabin's early versus late works. Consider Picasso's representational, blue, and cubism paintings. And what about the stark difference between the poet Walter Benton's erotic This is My Beloved and his bleak Never a Greater Need? Time, places, circumstances, outlook, focus and use of media all change.
So it's true that quality and appeal may vary. But if a composer were to spend a lifetime being consistent and predictable, I bet we would soon become bored too.
Topic: When your favorite composer dissapoints...