There's definitely something different in the way it's been implemented, like how it's displayed for example.
It's really hard to give any solid answers though as the default video player isn't open source, we simply don't have the source code available to study and compare.
Generally you'll find the default player looks better and is smoother for playback, when I watch a movie with the expectation of the best display quality available I tend to use the default player, when I want to be able to do stuff in the background or lock my phone and keep listening, for documentaries etc, I use OVP.
I personally consider OVP as a companion to the default player as opposed to a full replacement.
It's unreasonable to expect anything from software that's built in a developers spare time and released for free, essentially as a gesture of goodwill.