@arspr, To add a bit more detail to what theducks said, the @font-face code
Code:
src: url(res:///Data/fonts/... ...)
is typical of an epub which has been specifically prepared to use on a Sony PRS505/600/350/650 device. It externally references fonts stored on the Sony itself rather than embedded in the epub.
I'd guess the user deliberately put them in there using the calibre conversion Look&Feel ExtraCSS option. IMO it's a bad idea to force device specific code into one's calibre master epubs. I would have guessed that it would be completely ignored by the Kobo rather than causing havoc, but as I haven't tested it myself I could be wrong.