I'm assuming that the publisher just ran some automated "stick a page id in here somewhere" program that looked at a print book and at the start of each dead-tree page, stuck a tag into the ebook. Probably 1) no human ever saw it 2) when they made the ebook there might not have been any standards, and 3) no one ever looks back at the horrible stuff they did in the dark ages to make it better.
Of course, on the glass half-full side of things, if they finagled those page id locations to be in the next space, then when someone referred to a bit of text by page number, an ebook user might not be able to find it. Although, occasionally being half a word off shouldn't be too onerous.
EDIT: From that "Page Navigation" link you provided, inline page markers are supposed to look something like:
"<span
role="doc-pagebreak"
id="pg24"
aria-label="24"/>"
Yet, I don't think I've ever seen anything like it. Ninety-nine percent of the time, it'll be the old <a id="pag_330"></a> method, which that document specifically says bad things about. Occasionally, I'll see something like what's done in the current book I'm editing ("<span epub:type="pagebreak" id="page_330" title="330"></span>") which seems to be making some kind of effort. At some point (probably about where I've finished re-formatting all the books in my library
), I might know enough about this stuff to have realized I should have changed all those references while I was in there fooling around.