No clue just been a very long known bug.. a dispenser won't react to a rapid pulsar, but if you apply a constant on signal then a rapid pulsar it shoots extremely fast. Most likely just a hiccup in the coding like all oddities in Minecraft.
the gist of it is, if a torch is providing a constant power source to the dispenser, it will shoot every time there is an update to a redstone wire exactly 2 blocks away. this means 2 horizontally/vertically and 1 diagonally, even if the wires are not connected to it.
http://tinyurl.com/LB-SSS
In every Pre-Release, There is ALOT of Bugs.
~TheObsidianCreeper
^^^ click the picture ^^^
pretty old
You have wires going to the Dispensers HERP-Derp