[DRG]

Becasue the "weapon" in this special case is the stielgranate which has a significantly larger warhead size than 37mm AP. I suppose it could be handled like we do the Russian ATGM firing tanks in MBT with the stielgranate on the second line and normal HE / AP on the first but it would require further testing and scenario use checking all of which takes time I could otherwise be spending on other things rather than a unit / weapon combination that has been set up like it is for 2 decades

As for that photo.....one of your other sources says that " may be a standard H.E. round or possibly the special blank launching cartridge"