Why is there two interstate 635?

There can be multiple instances of similarly numbered spurs or loops in the Interstate Highway System. Three numbers, starting with an even number, indicate a loop. The first number (2,4,6,8) is usually a measure of distance from the city itself. The last two numbers are the Interstate which the loop is formed around (e.g., I635 is a loop where you can enter from I35 on one side of the city and reenter I35 on the opposite side of the city).