The key to soft cotton is the length of the staple or strand. Different species of cotton produce staples of differing length. Shorter staples require more weaving together to form a long thread which results in a rougher fabric. Both Pima and Egyptian cotton have longer staples, but Egyptian is superior.
In fact, a 200 thread count Egyptian cotton is superior to an 800 thread count Pima cotton sheet. Interestingly, Egyptian cotton is actually a native American species that was introduced to Egypt during the American Civil War to provide Europe with a stable alternate source of cotton.