I don't know all the details, but numbers vary by geographic reason and the date when they are assigned.

The first SSNs were issued by the Social Security Administration in November 1936 as part of the New Deal Social Security program. By the end of 1937, over 37 million numbers had been issued.Before 1986, people often did not have a Social Security number until the age of about 14, since they were used for income tracking purposes, and those under that age seldom had substantial income. In 1986, American taxation law was altered so that individuals over 5 years old without Social Security numbers could not be successfully claimed as dependents on tax returns. The Social Security number is a nine-digit number in the format "123-45-6789". The number is divided into three parts. The Area Number, the first three digits, is assigned by the geographical region. Prior to 1972, cards were issued in local Social Security offices around the country and the Area Number represented the office code in which the card was issued. This did not necessarily have to be in the area where the applicant lived, since a person could apply for their card in any Social Security office. Since 1972, when SSA began assigning SSNs and issuing cards centrally from Baltimore, the area number assigned has been based on the ZIP code in the mailing address provided on the application for the original Social Security card. The applicant's mailing address does not have to be the same as their place of residence. Thus, the Area Number does not necessarily represent the State of residence of the applicant, either prior to 1972, nor since. Generally, numbers were assigned beginning in the northeast and moving westward, so that people on the east coast had the lowest numbers and those on the west coast had the highest numbers. As the areas assigned to a locality are exhausted, new areas from the pool are assigned, so some states have noncontiguous groups of numbers. Complete list of area number groups from the Social Security Administration The middle two digits are the group number. They have no special geographic or data significance but merely serve to break the number into conveniently sized blocks for orderly issuance. There is a theory that the two middle digits can be used to identify a person's ethnic background. This is debunked as an urban legend on snopes.com as well as on the Social Security Administration's website. The group numbers range from 01 to 99. However, they are not assigned in consecutive order. For administrative reasons, group numbers are issued in the following order: ODD numbers from 01 through 09 EVEN numbers from 10 through 98 EVEN numbers from 02 through 08 ODD numbers from 11 through 99 As an example, group number 98 will be issued before 11. The last four digits are serial numbers. They represent a straight numerical sequence of digits from 0001-9999 within the group