If I remember correctly, the 20 billion cells per gram claim comes from Jamil Zainasheff. While he has done a lot of work in the area of cell counts and yeast starters, he has not published any of it directly (some may be in the book he co-wrote with Chris White titled 'Yeast'). Given that, I take it as anecdotal and not a verified claim.
The yeast manufacturers usually err on the safe side for cells per gram claims, but I would think their counts are much more reliable in the long run. It is always good to double check the manufacturer's specifications, as some of the latest releases are lower (specifically, the New England East Coast Ale Yeast from Lallemand) based upon their data sheet.