Well I don't know, they could make it into a plot point that Anakin is being given a padawaan even though Obi Wan probably believes he's not ready. Maybe the clone war has wiped out so many Jedi that they can no longer assign padawaans just to Masters. Lucas isn't writing it or directing it, keep in mind, so I wouldn't rule out a thought-out justification of some kind. Whether its believable is another matter though.