Why is the measured distance uncertainty for [M33][1] (835 ± 105 [kiloparsec][3]) six times bigger than the measured distance uncertainty for [M31][2] (778 ± 17 kiloparsec)?

They are approximately at the same distance, are both spiral galaxies and in the same region of the sky (within 15 degrees).

  [1]: http://en.wikipedia.org/wiki/Triangulum_Galaxy
  [2]: http://en.wikipedia.org/wiki/Andromeda_Galaxy
  [3]: http://en.wikipedia.org/wiki/Kiloparsec#Parsecs_and_kiloparsecs