What exactly is the difference?
Why did Europeans (who would know a bison by the physical similarity to bison in Europe call the North American bison a buffalo?
The European Bison Bison bonasus, or Wisent, is the same genus as the American Bison Bison bison. They are so similar they are sometimes considered to belong to the same species.
Buffalos include the Water Buffalo Bubalus bubalus , originally of south Asia, and the African or Cape Buffalo Syncerus caffer, which are rather different animals.
As to why the first explorers made this mistake, they were not naturalists and often called animals in the New World by the name of a rather different one they knew in the Old World. For example, they called the Wapiti, a large deer very closely related to the European Red Deer an “Elk,” even though this name properly belongs to a Eurasian animal closely related to the North American Moose.
Don’t even get me started on how badly they screwed up bird names.
Shouldn’t be too surprising since the first European arrivals couldn’t even get the name of the ***people ***in the Americas right. 
I forgot. What is the signifigance of a binomial that’s the same, like Bison bison or Gorilla gorilla? Other that I’ve decided that Bubalus bubalus is fun to say. (Hey, think if I say Bubalus three times a water buffalo will appear?)
It usually means that at the time of the naming it is the first and only member of the genus that has been identified. But there is no rule that says you must name a species that founds a whole new genus in this manner.
What’s the difference between a bison and a buffalo?
[Aussie accent]
You cahn’t wash yer fice in a buffalo!
[/Aa]
What is the connection of the other “buffalo” of the Malay Archipelago to B. bubalus? I know there are different species endemic to sevral of the islands.
Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo. (Scroll down.)
Sorry, couldn’t resist.
Mushroom mushroom!!
Beautiful link, thanks!
Right. Such “tautonyms” are permitted in Zoology, but not allowed in Botany (the Zoological and Botanical Codes of Nomenclatute being separate).
In the case of the two Bison, they were both originally described in the genus Bos by Linneaus (the same genus as domestic cattle) in 1758, as Bos bonasus and Bos bison. In 1827 the separate genus Bison was set up for the two species, so that the American one’s name became a tautonym. In fact, the American Bison has two subspecies, Bison bison bison and Bison bison athabascae, the latter the Canadian Woodland Bison. Some taxonomists still believe they should be included in the genus Bos.
Besides the Water Buffalo itself, there are three other species in the genus Bubalus, the Tamaraw B. minodorensis on the island of Mindoro in the Philippines, and two dwarf species, often separated in the genus Anoa, in the lowlands (B. depressicornis) and highlands (B. quarlesi) of Sulawesi (Celebes).
More information on wild cattle can be found in my Staff Report: What did cows evolve from?
The water buffalo does not actually have a repeated name. Look closely: Bubalus bubalis.