Errrr ... yes - I meant so say: "... an octave lower than the G row of a G/C Anglo ... "! (That's the problem of responding on my very dodgy home dial-up ... it falls over so often that I don't spend enough time reading my own messages - and that sort of thing slips through!)
It is one of the oddities that a player of both button accordion and (Anglo-)German concertina has to absorb: a 2-row button accordion (other than semitone-spaced chromatic types) has the inside row a fourth higher than the outside row: say, c above G. A 2-row (Anglo-)German concertina has the inside row a fifth below the outside one: say, C below g!
I suppose the original reason may have had something to do with the size of reeds that could be crammed into a little concertina body (or maybe not ... perhaps German concertina makers were at a different end of the Klingenthal to the accordion makers ... ?). When the English picked up the instrument, they kept the German tuning scheme ... and players of both instruments had to learn to mentally change gears when they changed instruments. As long as you just played within single rows, there was no problem, but any 'cross-row' techniques you picked up on one type of box ... didn't work on the other kind!
BTW: One of the nice things about a D/G (at least, to my taste) is that it is a fourth lower in pitch than the more usual G/C. I built my own (with concertina maker Richard Evans looking over my shoulder) from a surplus G/C ... switching reeds were possible ... drawing on Richard's stock of lower reeds where necessary ... opening out reed slots where the reed shoes were now too big for that position. The result is very nice - if not as sonorous as my previous D/G, for which Richard made up new steel reeds, utilising the reed shoes of an old set of brass reeds. Unfortunately that one was stolen from my car when I foolishly left gear in the back during a music workshop ... so I had to build another!
I like lower instruments, especially when accompanying others - on stage or at my workshops. I have an older Italian Bastari G/C (now called Stagi) in "Organ Tuning" - two reeds per note: standard pitch plus one octave lower. This doesn't have the "sweetness" of the best English solo instruments ... but it adds a lovely low-end 'grunt' to ensemble playing - and rattles out some lovely European polkas and mazurkas!