Why does the IRS issue a penalty for having *unequal* quarterly tax payments, even if my quarterly payments correctly reflect owned taxes for that particular quarter?

I don't have a crystal ball as to what my employment status (currently not working), dividend/interest amounts will be, if I decide to do Roth IRA conversions this year, etc.  I'm happy to pay taxes for the quarter that has just passed, but the expectation of me being able to predict my future income and set an equal tax amount to pay per quarter starting this month (when the 1st set of quarterly taxes are due) for this entire tax year seems odd.  What am I not understanding?