To sum up, Austin was overmarked but should have got a 10 from Len for the PD; Rachel was overmarked for her waltz but undermarked in comparison to Christine, who was marked equally with Rachel by Craig - so that is either overmarking or undermarking depending on how you look at it. Tom was undermarked but then has been all series, apart from those times when he has been overmarked. Lisa was overmarked so she wouldn't be in the dance off but she was the best dancer on the night, so wasn't overmarked in comparison with the others.
The judges should mark down if they see a fault or suspect there may be a fault but cant see; but should also not mark down if they cant tell there is a fault or if the fault doesn't detract from the dance. And the judges are doing this because they want to fix the leaderboard, they obviously have their favourites and they want Rachel/Tom/Austin/Lisa to win.....I think I've covered everything there.