Look at C2:C4:

C2 is 3% - 0% (0% is the rate for the non-existent 0th tier)

C3 is 5% - 3% (3% is the rate for the first tier)

C3 is 7.5% - 5% (5% is the rate for the second tier)

It is the difference between tiers. I don't think I can explain it any other way.