That information was calculated using medieval demographics, so there could be a problem with my reference materials (always a possibility when dealing with historians) or it just could be that cities were extremely crowded and houses were much smaller during that time period than we are used to today.
Also, the number of structures does not necessarily mean buildings - it also includes the number of fountains, statues, sheds, street stalls, etc. Those are also average numbers. Put in a graveyard, and the average number of structures goes up because each headstone/grave is a structure - and a lot of them can fit on an acre.
I don't reccall this portion of the book, but English waddle and daub houses in the middle ages were often 8' x 12' (or smaller) to house up to 10 people, with a fire going in the center of the hut. It may or may not have had a ladder to something like a loft above.
DnD has really messed with our ideas of dwellings in early history games' sense. So while the historical references Vincent mentions may have been faulty, they probably weren't I'd guess. (Now, I'll just have to try to look this section up tonight I guess!)