A command block can power a redstone comparator facing away from it (possibly separated by a block) with a signal strength specific to the last executed command (for example, the number of players affected by the command).
The signal strength always reflects the last command executed, even after the command block is deactivated.
According to the output in the chat, 3 entities were found. So I would have expected the output of the comparator to be 3. Even if I take the wiki description literally ("[...] number of players affected"), I would have understood an output of 1. But it is 4 instead. How? Why?
OK, I feel stupid: the comparator's output is actually 3. But, in my setup, the 4th redstone lamp was weakly powered by the 3rd one, which in its turn was strongly powered by the redstone dust on its top (well, I think).