Strange, I do this all the time.  One of my standard late-game tactics is to put vasari SBs at each of my allies' key planets, along with the occasional minefield; when the peace inevitably fails, my fully-upgraded SB (2 attack, 4 defense, 2 deflector) often wipes the floor with the SBs they have (as other players, human or AI, often buy the economic/support abilities for bases at their own worlds), and it then proceeds to destroy everything else in the system... even if they bring in a fleet to defeat it, the SB will do a LOT of economic/battle damage before it goes, and it'll take time.

But to the OP, I'm not sure how starbase arcs are defined, but when a capship says it does X damage, that's the sum of its various banks of that type, each of which will have a different firing arc.  So a capship that says 70 laser damage might have two 20-point banks on the sides and 30 on the front.  In this case, I'd assume then that while the starbase lists at over 100 base damage, a good fraction of that could be in banks that weren't able to fire on the enemy SB, hence the amounts you saw.