Grr.
I was doing some checking of scope math and the results from manual calculations using Cursors didn't quite jive with the scope's automatic Measurements. Looking more closely it seemed that the Cursor positions were being reported a little off. So I checked it out without any waveforms on screen to complicate matters. See the screenshot below for the errors in the Vertical Cursors (horizontal lines measuring vertical values). The CH1 baseline marker is exactly on the screen centerline (0.000V offset) and the Cursor B is also positioned right on this line, so it should be reporting 0.000 V but it actually reports -100.0 mV. The Cursor A is positioned exactly on the -10V line but the position is reported as -10.20 V.
This is right after running a Self-Calibration routine, too. And I get similar results from using other channels and different vertical scales. The Cursor positions are reported incorrectly! Not by much you might say, but enough to throw off calculations by a significant amount.