For those of you not familiar with the gory details of PDF, it basically uses absolute positioning for each character. If we converted that directly into HTML it would be a disaster. So, we actually extract quite a bit of structure on top of that, recognizing spaces, lines, columns, and paragraphs, which enables us to write much cleaner HTML. Scribd (and most PDF readers) does this with heuristic algorithms that make reasonable guesses.
In the future, we'd like to push those algorithms further, and extract ever more semantic markup. But this is a "nice to have" for us - mostly, people just want the documents to display correctly and load quickly. And, anyway, expecting the output of an automated converter to match what a human would write shows a basic ignorance of the state of computers and AI.