When you run a composition in Vuo, multiple nodes can execute at the same time. This takes advantage of your multicore processor to make your composition run faster.
In this composition, the two Count nodes are independent of each other, so it’s OK for them to execute at the same time. When the Fire Periodically node fires an event, the upper Count node might execute before the lower one, or the lower one might execute before the upper one, or they might execute at the same time. It doesn’t matter! What matters is that the Add node waits for input from both of the Count nodes before it executes.
The Add node executes just once each time Fire Periodically fires an event. The event branches off to the Count nodes and joins up again at Add.