Droppers and dispensers only react to rapid one-tick pulses if the signal strength is below a certain threshold (or the redstone wire length is above one, depending on how you word it).
This video I recorded shows the issue more clearly, in addition to the screenshot.
http://www.youtube.com/watch?v=RxOUAR8Q5V8
Linked issues
is duplicated by 26
MC-71010
One specific comparator timer doesn't power command block
Resolved
MC-56298
Command blocks not executing while triggered by a very fast clock
Resolved
MC-25797
Command block not updating
Resolved
MC-12203
Clock bug
Resolved
MC-11494
Comparator only graphical redstone flickering.
Resolved
Attachments
Comments 7
Now that I think of it, that works as intended, let's see how this comparator clock works: 1. Comparator is in subtraction mode.
2. You provide a signal from the back, comparator outputs strenght of 15 in the front. This loops to the side where the signal is 13. Comparator is in the subtraction mode: signal A - signal B. In this case signal A is still 15 but B is 13. So now the output of the Comparator is 2. That's why powered mechanisms won't react because the signal does not shut off completely but goes down to 2.
Looks like you're right. My initial tests using comparators as fast clocks (attached screenshot "othertests") had the pistons placed slightly differently.
Here's a quick overview screenshot.