why getElementById It returns different every time in console

I'm new in js and i'm wondering why every time getELementById() returns different in console for example : index.html : The message script.js : let h1Elem = document.getElementById("header"); console.log(h1Elem); result in console for sometimes : The message and sometimes : h1#header accessKey : "" align : "" ariaAtomic : null ariaAutoComplete : null ariaBrailleLabel : null... By definition of W3School I expect getElementById() it to return an element with a specified value, but sometimes it doesn't