Without doing any research, I seem to remember that historically the Army had Lieutenant-Generals and Sergeant-Major Generals. The 'Sergeant' got dropped off, but the 'Major' stuck, so a Lieutenant General is a 3-star, but a Major General only two. - Danny
As you are wont to say, YLSNED!
On mature reflection, it's probably best not to reflect on how the ranks of Vice and Rear Admiral originated....
Jack