What does this mean?
It means, the biggest discount you'll get is 21 agurot, and that's from full service price.
If you fill up in your own you'll get the price of full service minus 21 agurot, which is less than self service regular price.
Let's say the price of gas is 7.50 per liter full service
So usually the price of self service is 18 agurot less, so you pay 7.32.
With this discount you'll pay additional 3 agurot less.
Basically you'll pay 21 agurot less from the full price no matter if it's full service or self service.
Btw if you want the Israeli CC just for the sake of discount on gas, there are gas stations in Israel where you pay less to begin with, and comes out cheaper than any of these "discount" offers.