Piston double pulse

Whenever I try to make instant wire with the set up that's in this video (by Sethbling, the set up is at 11:18):
http://www.youtube.com/watch?v=tT_nwbqMYgU&lc=c73cdzB2M2_DrWSOlfx9ef9zmCB6JXdql8QIdHODOKY

I sometimes get pistons that will pulse 2 one-tick pulses instead of just pulsing once, like they should (according to the video). It all depends on where you place the piston and what orientation you place it. I don't know if it has something to do with it registering the different changes of signal strength in those 2 ticks and/or it (just) has something to do with the redstone mechanism orientation glitch.

To re-create, simply re-make the instant wire that's shown in the video and you'll eventually come across a double-pulsing piston.