Dispensers on a clock acts unpredictable

Dispensers powered by 2-tick clock.

What I expected to happen was...:
All the dispensers should fire.

What actually happened was...:
Some of the dispensers fire, others only fire upon block update.

Steps to Reproduce:
1. Redstone wired into a repeater.
2. Repeater goes into a comparator set to subtract
3. Comparators output signal goes into repeater on the right side of the comparator
4. Run the comparators outputted redstone signal on blocks above the dispensers.