It’s a tiny new feature, but you’ll be amazed at how quickly you start using it. Release the mouse button, and the three cells are no longer part of the selection.
As you are dragging, the area becomes a lighter shade of gray. With the mouse, click on M8 and drag it to M10. With this new feature, however, you can simply deselect the unwanted cells.
You had to start over again (and likely did it more slowly to make sure you didn’t slip again).
Once you released the mouse button, they were part of the selection. But when you did the same with cells I8:L10, you went one column too far and inadvertently selected cells M8:M10 as well. You then held down Ctrl and used the mouse to select B8:E10, then I2:L4.
