Instead of using H1, H2, H3 many use a P tag with a gosh-awful CSS class call out.
H* tags always trigger a page-break-before on the Kindle. That's not always desirable. Like chapter numbers with a (same-sized) chapter name beneath it. Nobody wants a pagebreak in between those two items, so they use a P tag with CSS to simulate a header.
chapter headers were more consistently tagged, but I also understand why they're oftentimes not.