Mac OS battery cycle count
From Apple's support note on battery cycles:


  A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could use your notebook for an hour or more one day, using half its charge, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so it may take several days to complete a cycle.


So essentially, as long as you ever run the laptop unplugged at all, your cycle count will slowly increase.