In the 3rd Imperium Universe, there seems to be a hierarchy of responsibility with space based security. While a subsector navy makes its presence know when necessary, they only get involved locally when its significant to the Powers That Be. It's the subsector navy that receives pooled research and production in return for mutual defense.
Local defense is the responsibility of individual systems unless there is an specific mutual pact such as a local ruling family in control of several worlds. As mentioned above, megacorps can have a stake in one or more systems that would be cost effective to defend. This would explain the variation and sophistication of system defense, if any at all. SDBs on worlds with the resource, credits and manpower will design, build and maintain their own forces shaped to local conditions.
One exception, and fitting in with larger corporations, would be worlds unable to create their own but having a need for protection. They would lease SDBs shipped in from elsewhere and maintained from tenders either long term or temporary. Excellent for colonies. You could enter a system with antiquated vehicles meant more as a minimal Rent-a-Cop show of force.