That makes sense - looks like WebM/VP8 support in browsers is a superset of Ogg/Theora support [1]. It would be silly to serve up Ogg/Theora unless it yielded better quality/bitrate (and I'm not up-to-date on how those two compare in quality at the same bitrate).
> Note that there are 6 separate encodings -- mp4, webm, and reduced mp4, for forward and reverse