Why does attributeChangedCallback fire for attributes already present on a custom element when it is upgraded, but only for observed ones?

<x-el data-a="1" data-b="2"></x-el> <script> customElements.define('x-el', class extends HTMLElement { static observedAttributes = ['data-a']; attributeChangedCallback(n, o, v) { console.log(n, o, v); } }); </script>