Good question ... in that one needs to ask why flush rotation differs.
And from that comes other questions ... such as ,,,
What makes the Earth rotate?
What makes the ocean's tides?
What makes the Earth's magnetic poles reverse "every few hundred thousand years"?*
What makes the Moon-face toward the Earth constant?
* The pole reversal fact comes from the book Darwin's Ghost by Steve Jones ... page 252.
What makes the ocean's tides?
What makes the Moon-face toward the Earth constant?
I think Earth's rotation is caused by the moon's pull on Earth's oceans.