Pistons behaving (slightly) abnormally

I was doing random redstone stuff when I saw something abnormal. When a diode is powering 2 pistons (I was using sticky ones), they 2 stacked 1 onto the other and facing the same direction, the botton piston extends sligthly slower (probably 1 tick slower) when it should not. I repeated the conflicting part, some blocks away, and it then worked normaly (may mean changing the chunk where the circuit was, fixed it...).