Why do I sometimes see leaves but no Pokemon appears?

The leaf animation signifies a Pokemon is at that location, but there are many times that I stand directly on the animation for multiple minutes and nothing pops up. I've tried walking around the spot and still found nothing, so it doesn't seem to be that my location is slightly off. Some other possible causes that I've ruled out:

• It cannot be a Pokemon that I already caught because it happens in areas where I have not caught anything at all.
• It cannot be an old Pokemon that's despawned before I got to it, because it sometimes happens even after restarting the app.
• It doesn't seem to be a poor connection because I can (sometimes) transfer Pokemon, spin spots, catch other Pokemon, track distance, etc. with no issues.

I've also noticed that there are no Pokemon in the "nearby" list with zero steps when this happens, even after restarting the app. There isn't always one listed even when I can see them, though, so I don't know if it is related or coincidence. My last idea is that it's a Pokemon I'm too low level to catch, but there's usually no one playing by me when this happens, so I don't have any evidence one way or the other. Does anyone know the reason this happens, or should I chalk it up to general bugginess?

Solution 1:

Borrowing from @Codestrife's answer here

The rustling leaves usually indicate that there are pokemon near that area. Going and walking around near that area gives you a higher chance to encounter Pokémon.

Keep an eye out for rustling leaves, hinting at where critters may be lurking and simply hang out in a suspected region for the increased chance of an encounter. - Source

Therefore, the reason you don't always see Pokémon in rustling grass is because there is only a higher chance to encounter them there and not a guaranteed chance of encountering them. You just have bad luck when you don't encounter them.