Here’s the SSA’s official explanation of its numbering scheme. It says the numbers are assigned based on where the application was processed, and that one did not have to live in the state where the number was issued.
It also says the final 4 digits are sequential, but I’m not sure it claims they are assigned sequentially, as that is far from my experience with my kids.
http://www.ssa.gov/history/ssn/geocard.html
didn’t Orly Have court case saying he had a number of SS numbers that he was using