If you did happen to find one that was 20/1 + generally the owners and trainer thought it was little hope as well,
Not all stables are big punting stables Gallopers, we have had plenty of runners we thought could win at good odds, and have had plenty that have.
I don't know about maidens with unexposed form , but if you look right into any horse that wins at good odds you can normally go back and find a reason to have backed them.
Most of the time people don't go back far enough into their form and, as you suggest don't put all the pieces of the puzzle together.
Thing is if you go back though every horses form you could probably find a reason to back most, but it is the real obvious reasons that some horses start alot shorter then most.

edit: disclaimer, there will always be one that wins with out any reason at all
