Read the TOS. Your daily budget will be averaged over 30 days. So if it is $20, you may have days it will be $21 but to compensate, you will have another day it will stop at $19. But over the month, Google guarantees you will not go over $600 ($20 x 30). It's all to maximize the number of clicks you get, plus the fact that making you spend exactly $20 each and every day is near impossible. I suggest that if you are reaching your budget every day that you increase it, as long as your ROI is good.