That is a good question! You kinda answered it in your note.

The reason you can get an accurate measurement from a corner or an edge is because of the movement.

When you measure from a corner your measurement starts on the outside edge of the hook.

When you measure from an edge, the measurement starts on the inside side of the hook.

The hook is about 1/16" inch wide, so the error between the two measurements is 1/16".

If the hook moves, say 1/16" it will correct for that error.

Does that help?