the rules state that you owe 1/4 of your taxes for each period (quarter) regardless of actual liability for that period. that's the default. so the IRS/Turbotax don't have to know what each quarter's income was. there is an optional Annualized Income section on the 2210 which if you go through it your liability is determined by period based on income/deductions/ additional taxes/tax credits/withholding and estimated payments for each period.
