What's the difference of the two systems?

Solution 1:

Ok, so finally I found the reason by myself.

Before I start to explain, I forgot to say that that machine was supposed to do: displace or drag back the block in a location, and to avoid the observer block doing an entire mess, we have to retract the pistons in a specific order.

To make them retract in a specific order, I had to use a comparator. The thing is that the comparator compare which side's signal is the strongest, and if they are the same, it will let the signal pass, which is EXACTLY what I DON'T want.

So what I had to do is simply to switch the comparator from compare to sustract mode, since 15-15=0.

PS: Because the problem is solved, I won't put a world download.