There’s a chance 
We had a user in the USA who was being overcharged - they got a refund based on OEM data.
It all done in PHP. The Input processes execute as the data is received. Each process is executed in sequence - top to bottom, the Input value enters at the top of the list and falls into the first process. That works on it and either passes a modified value out of the bottom, or sends the value or a modified one out to a Feed and passes the unmodified value down, to be picked up by the next process. (The blue ‘help’ note tells you).
Your problem was the original value had been modified, so you needed to start again with a reset. Equally validly, but maybe not what you wanted here, you could have reset to zero and then added the input - or even a different input.
Because of timing considerations, it’s generally better to add a Feed, rather than the corresponding Input.