The Mississippi Territory was divided evenly to create the states of Mississippi and Alabama. The demarcation down the center was intended to create two states of approximately equal size and equitably split the valuable agricultural land in bottom of the states. That was an intentional decision by Congress to ensure new states of Georgia, Alabama and Mississippi being roughly equal.

The "tabs" at the bottom were not in the original maps of the territory, because that land belonged to West Florida, which was controlled by Spain. The United States Congress secretly authorized the President to militarily seize the land that became the tabs touching the Gulf of Mexico. Using this authority, General James Wilkinson, senior officer of the United States Army under President Thomas Jefferson, occupied this district with a military contingent; and the Spanish colonial commandant reportedly offered no resistance.

Oddly enough, Wilkinson was secretly working with Spanish colonial officials to help contain American expansion. He received payments and land from Spain for his efforts, which included attempting to prevent Kentucky from becoming a state, and reporting on the purpose of Lewis & Clark expedition into the Louisiana Purchase Territory.

Sources:

• How the States Got Their Shapes by Mark Stein.

• Library of Congress video of Mark Stein discussing the shapes of Mississippi and Alabama. Cued to relevant question/answer.

• An Act authorizing the President of the United States to take possession of a tract of country lying south of the Mississippi territory and west of the river Perdido

• General Wilkinson and his later intrigues with the Spaniards by Cox, Isaac Joslin, 1873-1956