When I am not accessing a unit on the Flow, how many times has this unit been used?

that Add unit has var1 input 

Twice, though since there are no side effects, the same value is computed each time.

See here for a bit more information:

https://support.ludiq.io/communities/5/topics/1080-control-flows-and-caching

Basically, because it does not have a control flow, the value has to be recomputed because the input might have changed between executions.

Indeed! If you want to enforce it being calculated only once, use the Cache unit.