What is supposed to happen is if a ship type A has, say .1x the cap of ship type B, then if the AI chose to reinforce with ship type A, it would only get only .1x the number of them as they would of gotten if they chose ship type B. So what could of happened is that the AI may have chosen SSB for each of it's reinforcement cycles, thus, even though they don't get many (often times, only 1, or less if it had to spread the "reinforcement points" around) per reinforcement "pulse" to that planet, over time, that could add up. If they had chosen, say, missile frigates for all those times, you would of seen that 700 or so missile frigates on that planet.
However, there have been bugs and oversights in the past that let the AI violate this tradeoff of numbers.
Even if the logic is working properly, it may be that the cap per planet and/or the cap per guard post for certain ship types needs to come back, to prevent alerted planet to just "sidestep" the max defending ships total (across all ship types) per planet rules by just always choosing the low ship cap ships.