My sisters were born after 1990, and they have random ssn's ... especially compared to my brother and I who were born before 1990 (we were all born in the same

My sisters were born after 1990, and they have random ssn's ... especially compared to my brother and I who were born before 1990 (we were all born in the same city). I think the scheme implemented in 2011 is a more random one than the previous scheme or maybe it wasn't implemented everywhere before 2011, and took 20 years to implement (which is also believable).