Topic vs. Category

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If we post a question on the Q&A page we first choose a CATEGORY. If we suggest an idea on the Ideas page we choose a TOPIC. I can follow a Topic on the ideas page and I can follow a Category on the Q&A page and they BOTH end up on the "Topics I Follow" list on the MyFeeds page. Even the little icons match. So, if a TOPIC and a CATEGORY are the same thing, why the inconsistency? If they are different, why do they have the same logo/icon?