Thank you for your response mr_shiftright.
However, the numbers for with-the-seat-down are separately listed, in most cases (including Edmunds). The numbers for with-the-seat-up are distinct.
And yes, I could go measure it, but trunks are of course not square (especially in the case of the Avant with its sloping rear), so it would be difficult to tell volume exactly, plus I was trying to avoid another trip to the dealer just for this! If I do go, I will post my results as best I can calculate. I think the point remains that there is a possible inconsistency of measurements here that is making it difficult to compare the cargo space of the Avant to anything else-- or it really is small!
Thanks again for your interest and your help.