When you testfor using a command block, an adjecent comparator outputs a signal if the test for was successful. If you use the execute command on an entity and use testfor the comparator always outputs a signal (given the execute command succeeded). However, the previous output of the command block shows the testfor return as if it were running the command itself. Comparators should output the success of the command that the entity runs not the success of the command block's execute command.