Fixed chance ones are not affected by the combo stat. It is just a specific (non-disclosed) rng value.

Non-fixed chance ones are more likely to trigger as your combo stat get higher; and less likely to trigger as your opponent's combo stat get higher. If your combo stat is way higher than your opponent; a non-fixed chance event will almost always trigger. If your combo stat is far below your opponent; a non-fixed chance event will almost never happen.