I think I might be misunderstanding what you said; you say you "don't do anything special to avoid swarming" but I assume you mean you give them plenty of room and do an artificial split when (not if) you see the first queen cell?
It's my understanding from what I have read, and especially from what I have been told here in Ireland and seen from my first years experience - that each and every one of a beeks hives WILL make an attempt to swarm, only very small or sick colonies will not (the majority of the bees are 90%+ AMM, local stock if you know what I mean...), once a beek sees a Queen cell, do an artificial split and that should be it, if they attempt it again they are defined as swarmy and depending on who you talk to the general consensus is that you should re-queen (kill the queen and re-queen from another hive). My own nuc tried to swarm when they were filling up a brood box, they still had a lot of room.
Having re-read the above, I've only just remembered you use queens which are reared in spring, with each hive being re-queened each year, which I have read often leads to no swarming for that year only (my nuc had last years queen).