Because of pellets overall weird shape, when compared to a bullet, you can get what's called transonic airflow over the pellet, which makes for poor accuracy.
Essentially, because the pellet is fat/thin/fat air will move fast where it's being pushed apart and slow where the pellet is narrow. Locally some air will be moving faster than the speed of sound and some air will be moving slower.
When this happens it sends shock waves through the pellet and the surrounding air in unpredictable ways. The pellet does not have to go supersonic, just near supersonic, which is actually the worst. Shoot pellets that go slower than 950 fps to be safe.