In this composition, the Add node executes each time either Fire Periodically node fires an event. If one of the Add node’s inputs receives an event, it doesn’t wait for the other input. It goes ahead and executes.
If the two Fire Periodically nodes fire an event at nearly the same time, then the Count nodes can execute in either order or at the same time. But once the first event reaches the Add node, the second event is not allowed to overtake it. (Otherwise, the second event could overwrite the data on the cable from Add to Display Console Window before the first event has a chance to reach Display Console Window.) The second event can’t execute Add or Display Console Window until the first event is finished.
Compare this composition to the one above it. Since in this composition the Fire Periodically nodes can execute in either order, or at the same time, the results are unpredictable. When you want to ensure events are executed by separate nodes at the same time, use the same event.