The suggested (not obligatory) user agent styling for <b> is `font-weight: bolder` an agent or authors could use lots of different things to bring attention to what the element contains and treat it differently from <strong>.
https://html.spec.whatwg.org/multipage/text-level-semantics....
https://html.spec.whatwg.org/multipage/rendering.html#phrasi...
The difference, of course, is that one is a Latin word, and the other is an English phrase with the same meaning. But they're different words, so different tags. It's all completely rational and logical, and if you don't see the logic and/or reason, well: you're unworthy to write HTML. See Figure 1.