Even when "idle", the comparators update things 2 blocks in front of it and 1 block behind it.
It seems to be doing it every redstone tick(i think) (2 gameticks) which can cause unecessary lag and problems because dispensers can fire on updates etc.
If you flick the lever in the picture the piston will extend because its being updated by the comparator.
Ok, so i actually checked through the code.
Apparently, the comparator is updating itself + the blocks infront/behind when it gets updated. um, derp?