Hi Dan,
that's a good question. Hosted Libraries aims to host exactly the same files as provided by upstream sources and it seems like this might not have happened here.
My guess at what happened is this:
We take great care at Google to correctly document the license and authorship of any third-party code. Sometimes this means that we have to
add a license header in case the original files are missing one.
For Hosted Libraries we have an exception that we can serve unmodified copies of the original files, but maybe in this case the reviewer wasn't aware and a header was added by mistake.
I researched a bit about the extra license in that file. The Web Font Loader was co-developed by Typekit and Google.
I hope this helps! Sorry for the confusion.
Kind regards,
Philipp