What is with Undo?

What on earth is the Undo undoing? I make a whole bunch of sketch changes, and then I make a mistake and Undo and it undoes the whole lot? With no Redo?

What does the Undo undo back to?

In the most recent case, I made a construction rectangle, aligned it, locked a bunch of points, moved a bunch of points, deleted a bunch of construction stuff, moved on to the next change, made a small mistake, Undo, and it undid the whole lot, back before all the previous changes.

Version 5.770.0 (7944) / Mac