Because the bearing caps nearest the thread is different.
O.K, thanks for that.
So, if the bearing caps from your DX pedals were in decent nick, you could use them with the SX axles and they would look right? I thought you meant there was a noticeable difference at the flatted section where you screw them in with a spanner. (poor terminology, I know)
Sorry for being an anorak and a thread hi-jacker