10% of 100$ is 10$, so the new amount in the account is 100+10=110$. 10% of 110$ is 11$, so the new amount in the account is 110+11=121$.
20% of 100$ is 20$, wo the new amount in the account is 100+20=120$.
It's not the same, because when you do it twice by 10%, the value changes in between.