I found that if you drag a wooden block across a piece of 150 grit sandpaper, there is more friction than if you do the same with a piece of 50 grit sandpaper. Why is this? I tried to find an answer on the Internet, but they were not specific enough.
Short answer: finer grade sandpaper has more points of physical contact than rougher grade sandpaper, and each point of contact is a source of friction.
Extrapolate from there and see how it would apply to 150 grit sandpaper compared with 50 grit.