A comparator should always have a 1 tick delay no matter what's powering it.

Comparators that appear to be activated, but actually aren't, will in some cases have a delay of 0.5 ticks. This happens when they are powered by another comparator or a repeater. However, the delay is 1 tick when powered by a block or a redstone.

To reproduce this bug:
1. Set a comparator to subtraction mode.
2. Power rear input.
3. Power side input with the same signal strength as you used on the rear input.
4. Turn rear input off.
5. Turn side input off. The comparator should now look activated while receiving no input.
6. When this comparator gets powered by a repeater or another comparator, the delay will be 0.5 ticks.