[michaelt]

Most modern video container formats support arbitrary frame durations, using a 'presentation timestamp' on each frame. After all, loads of things these days use streaming video, where you need to handle dropped frames gracefully.

Of course, not every video player supports them well. Which is kinda understandable, I can see how expecting 30 frames per second from a 30fps video would make things a lot simpler, and work right 99.9% of the time.