The long (British) and short (American) ton are both 20 hundredweights. The American hundredweight is exactly 100 pounds, while the British hundredweight is 112. You tell me which of those is more reasonable.

That said, both units did, in fact, come from Britain. The old Imperial system often used the same name for different units depending on what was being measured and for what purpose. Both countries passed laws to simplify and consolidate these measurements in the early 19th century, but in many cases chose different versions to standardize on.