Unless you're streaming video games (high FPS, as you noted the inverse, and significantly larger portions of the screen being refreshed, usually the entire one) the 'deltas' are relatively small. Combined with low frame rates, your usual business presentation occupies a small share of the bandwidth.

on the other hand, I'm mostly staring blankly at my webcam with a static background. though admittedly the webcam sensor is a lot noisier :)

but yeah, I suppose the update rate of most screen sharing is pretty low.

It's usually pretty easy to compress because you're so often sharing text on a solid background. I'm figuring that there's gotta be a higher bitrate for sharing, say, a movie or video game than sharing my jira board.