I've had an email from EBC about why they make different brake pads for the rear 300mm and 310mm brakes on the Superb MK3.
Apparently, although ATE make the same brake pads to fit either size of rear brake discs, when EBC made their aftermarket copies they purchased OEM Skoda rear brake pads...and they were slightly different from each other. So EBC copied both.
EBC say that you are likely to be able to use either DPX2153 or DPX2173 rear brake pads if you are using new discs. But if your discs are used, it will be best to use the same pads that you are replacing (or at least have the friction material in the same place) because the friction material on the pads is likely to have worn an ever so slightly different track on the discs.
I think it is for this reason that some people have had problems (possibly vibration) with various cars when they fit new but different pads to used/worn discs, ie. there can be problems if the friction material on the new pads is in an ever so slightly different position to the friction material on the old pads.
In making different rear brake pads for the 300mm and 310mm rear discs, EBC appears to be the exception rather than the rule, as most brands make the same rear brake pads to fit both the 300mm and 310mm sizes. These brands include ATE, Bosch, Brembo, Ferodo, Hella/Pagid, Jurid, Optimal, Textar, and TRW. So out of these ten brands, it's only EBC that makes different rear brake pads for the 300mm and 310mm rear discs.