(no subject)

Is using the verb "deduct" to mean "to reason from evidence to a logical conclusion" a British usage thing? There's a scene in the NuWho S2 episode "Fear Her" where it's used in the present progressive, "Are you deducting?" and I've just seen someone use it in the same way in a Potter fanfic. To me those sound odd and the verb for this meaning should be "deduce", while "deduct" means "subtract" (although the noun can be either - a tax deduction, a sensible deduction). They come from the same Latin root, but -- mrr?