Upon further investigation it only seems to do this with PDF documents; the author and title fields seem to map correctly against html, zip and text based files.
So if I rename a pdf, an html file, a text file and a zip all to the same name:
wibble - wobble.[pdf|zip|txt|html]
...then the html, text and zip version of the file will all correctly display as title="wobble", author="wibble".
However the pdf file will show as title="wibble - wobble" and author="wibble".