I don't think the USPTO should be guessing in the first place.
H.264 is open, anyone may contribute to ISO/ITU projects (including those holding patents). WebM is closed, it was developed and is owned by On2/Google.
H.264 costs money to license and deploy; it is not free. On2/Google are currently the only bona-fide identified owners of the technology in WebM (as far as I know), and they do not charge for it.
There are open-source implementations of H.264 and other royalty-bearing codecs. I am aware of at least one codec (which I can't name) which had a free license but for which the source was only available on agreement; it was closed-source.
So, open/proprietary usually refers to the development process and the ownership and control of the specification. Open-source/closed-source refers to the availability and visibility of the source code. Royalty-free/royalty-bearing refer to whether payment has to be made to patent owners in order to deploy. I suspect that almost any combination of these choices is possible.