This is a first deposit bonus where you know what you are getting irrelevant of the amount you deposit. An example would be if a casino offers you free $80 if you deposit at least $20. That will give you a bankroll of $100 to play with. But if you deposit $50, you still only get $80. Again, this is a mathematics thing. If you compare it to the match bonus of a different casino, you may find that by depositing the $20, you get more for your money. Here it would mean that you get 400%.
So, before you dismiss a casino that offers you a bonus which initially appears to be low, make sure you work out what that would mean if you make the minimum deposit required.