Yes, there is a difference in the screen resolution. And on Windows, I can resize the window as much as I like and have a rock solid spritesheet animation at all kinds of resolutions. It's only on iOS that it jitters regardless of the resolution. I've tried on a few different screen sizes, including iPhone and iPad resolutions.
MonoGame must be able to display a simple 2D sprite sheet animation on iOS without it being all jittery, or it would be unusable for 2D games, right? I must be doing something wrong, but so far haven't been able to figure out what.