Now I see what happened; it's a little different than what you hypothesized.
TurboTax allows for four payments (four quarters, and there's a button to add more). My four quarterly payments were entered in correctly. Somehow, I entered an additional (duplicate) payment. When you look at the list of estimated payments, it only shows the quarterly payments and not the additional payment! (It would have obviously been a duplicate.) It also doesn't show that an additional entry was made.
The total does include the duplicate, and it was happenstance that it looked like a transposition error.
Obviously, on the data entry sheet, it would be nice to have had some indication that there was more data than was shown on the screen!
