those links will surely help...
On a hunch I went to:http://www.youtube.com/html5
and I noticed a discrepancy between chrome and iron on that page.
Both support: HTMLVideoElement, Media Source Extensions, VP9, VP8, and WebM
But only google chrome supports H.264 while Iron does not support it. My guess is that the higher resolutions on those videos are encoded via H.264 instead of VP8 or VP9 (despite google's claim to convert the entire youtube library to it).