A system’s security rating is determined by the presence and size of the Nation-State’s armed forces in that system. A higher level is indicative that a large amount of military forces are currently deployed in that system, and thus is assigned a security level befitting that from CONCORD.
If I recall correctly, Sarum Prime used to be a 0.3 security status system until a large number of starships from the House Sarum fleet massed in the system, causing an alteration in the system’s security status as a result.