Underpayment penalty when getting a refund

In addition to regular annuity payments that had FWT taken out, I also had 2 large withdrawals from my IRA during the year.  Immediately after taking the withdrawals I submitted estimated tax payments for them in the same quarter as the withdrawals.  The end result is that I have an overall refund for the year.  Yet Turbotax calculates a tax penalty.  It seems that Turbotax (and maybe the IRS) only takes into account the FWT when figuring the penalty and not the estimated payments I made.  I realize that I can ask for a waiver or ask the IRS to calculate the penalty, but why do I need to do this?  Also in actuality I didn't realize that Turbotax had added a penalty since it doesn't jump out at you in Turbotax that they are adding it, so I ended up paying the penalty.