I’m not a developer here but they explained this to me once, something to do with simplified 4-note chord voicing to keep it from sounding too dense and cluttered. I’m not 100% clear on the music theory concept (and it’s stupidly hard to find a good source online) but something about how the most essential parts of an extended chord are the 1, 3, and 7 notes. So a 9th chord would be 1, 3, 7, 9; a 13th chord would be 1, 3, 7, 13. Not sure why an 11th chord turns into a slash chord with 1, 7, 9, 11 (e.g. a D11 would be C over D), but it’s common for songs to use that chord (though often they have the 5 too, so it’s a shame that note gets omitted in Hooktheory, and therefore I understand your frustration.)
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