It isn't that simple. I'm glad you found it by yourself. The problem is, that one would think, that logically the first approach should be right.
IF something is so do this ELSE do something else
But that logic breaks, if the event picks something.
Enemy is overlapping Pool <- this picks the enemy-instances that are currently overlapping Pool
Else <-this oviously doesn't pick anything. But in Construct, if no specific instance is picked the action will be performed at ALL instances of the object.
So, good work, that you solved it by yourself.