Comparators will randomly fire off a Full signal while reading a chest that does not have the adequate items to complete that signal whenever the chunk loads.

This one is totally random. I have a set of 6 chests being read from a comparator. 60% of the time when loading the chunk the comparators will fire off a full signal when it does not have the required items to fire a full signal.

As you can see in the pictures the redstone blocks that have been pushed down are the comparators that read a full signal and fired the regular pistons.

This makes comparators useless when trying to calculate how many items are in a chest for occasionally they will fire.