AFAIK it's one of those W3C versus WHATWG/browser vendor things. The spec has an algorithm, no browser actually implements it faithfully, and when browsers were still competing with each other nobody was going to break half the web to be spec compliant. The problem started decades ago (https://html5accessibility.com/stuff/2022/04/05/12-years-bey...).
If I remember correctly, W3C’s XHTML2 working group wanted a generic <h> tag [1], and WHATWG, focused on evolving HTML in a backwards-compatible manner, repurposed <h1> as a context-dependent heading tag instead.
