Your question is about linguistics rather than halacha. Many things misnamed but we use the names anyways. In many countries the “Labor” party names suggests they are a party of blue collar workers, when in fact they are a party of left wing pseudo-intellectuals while working class voters support other parties. We refer to someone who hates Jews as an anti-Semite, even though there are many people speaking Semitic languages including the Arabs, and most people in Malta and Ethiopia (and someone accused of being an anti-Semite can not defended himself by claiming to like Arabs). That is how language is. One should note that since one says kaddish for almost a year, as opposed to a month for other relatives, the person likely to be saying kaddish in shul is usually saying it for a parent, which is probably how the term developed.