The Final Four region match-ups?

[ABEHONEST]

Last year's Final Four had this 2 game match:
EAST met the WEST and the SOUTH met the MIDWEST.

This year is different? If so, why? The FINAL Four 2018 looks to be like this:
EAST/MIDWEST and the winner meets SOUTH/WEST.

I never noticed before if they actually switched region match-ups every year?