It's not that simple, take another look.
First quarter: I am expected to pay the difference between 3 months worth of tax (x90%) and 3 months of withholding.
Second quarter: it's true that I take my income for 5 months up to May 31, then multiply it by 12/5 (2.4) - so far, so good. But then I am expected to pay the difference between 6 months worth of tax (x90%) and 5 month worth of withholding and estimated payments. This results in a much higher payment than in the first quarter if I partially rely on withholdings.
