Why is the Deputy Corps Commander's rank lower than the Division Commander's?

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From what I see, the Deputy Corps Commander is usually a Brigadier General,
while the Division Commander is a Major General.
Shouldn't the Deputy Corps Commander also be a Major General to maintain proper status?
In the absence of the Corps Commander, the Deputy Corps Commander must command the Division Commanders;
but since there is only one Deputy Corps Commander and not multiple,
I am curious why they appoint someone who is two ranks below them.