Variations in regional terms in USA

I was born in East Tennessee and have traveled extensively in Middle Tennessee and West Tennessee. I live in South Florida and have visited Northern California. Does anyone know why there is a difference in the construction of the modifiers to the state names? East Tennessee, and so forth, are part of the Tennessee State Constitution.

Solution 1:

Tennessee is unusual among states; most geographical designations are not defined in law. For example, West Texas and East Texas have no specific boundaries, and people have different ideas about how a line between them might be drawn. This is true as well of Northern and Southern California.

Wikipedia notes about East Tennessee:

Unlike the geographic designations of regions of most U.S. states, the term East Tennessee has legal as well as socioeconomic meaning. East Tennessee, along with Middle Tennessee and West Tennessee, comprises one of the state's three Grand Divisions.

https://en.m.wikipedia.org/wiki/East_Tennessee

There are some geographical designations that cover more than one state, such as the Great Plains or Appalachia, which includes East Tennessee, as you know, and all or parts of other states. Southern California includes the high desert and the loe desert, designations that are sometimes used in weather reports.