Oh, those. I do not know what is your experience, but I found those turrets rather weak (they could hardly scratch my Freighter once; those were on a core2). They might be effective against fighters, I did not try that, so I think they are rather cosmetical than effective.
I believe it was a (unintended? ) side feature of the last pob revamp, in which new bases gained a set of guns on the main structure, while existing bases were untouched by this change.
Seems this might be your case, and probably won't change unless your pob, suffers a model change /refresh (via upgrading ).
In terms of defense value I dont see how these turrets can be any better than the satellites. Positioning them on top of the base takes quite a bit ofceffective range away.
Anyway, nice observation. nNever really realized these differences.