Raze_Larian: I don't know. That could have been part of it, or maybe it was just easier to make one version of the DLC... or maybe it is because GOG generally has soundtracks, when possible (and for D:OS the soundtrack is listed under extras, rather than the DLC).
But why would they need another DLC version? Backers get the trunks and the potion anyway....and yeah, of course GOG usually likes to have the soundtracks as a bonus, but it gets hard to do that, since soundtracks nowadays can usually be bought as an extra.
Again, it's not that I don't like the extras, but other people buying the game from somewhere else might complain (even if it's just for us GOG backers, then Steam backers will complain xD). You should try to check if it wasn't a mistake or something =)