The intention was that they do not stack.
The information surplus - overflow - of the internet could pose a greater challenge than the more limited resource of encyclopedia britannica … look for cats on the internet = LolCatz! In the encyclopedia = more specific. Larger amounts of data might not be so rigoursly tested for validity.
Still, for archives like this I guess one could make a good argument for a stacking function… and then, does the archive acutally contain the information you're looking for? if not, there is no bonus at all… it depends how you want to use it really. I'd go for non-stacking.