Most diesels have a return fuel line from the injector pump that by design pumps more fuel than needed for each firing. Especially the positive displacement pumps. These push out a certain amount of fuel on each stroke (amount controlled by throttle setting) and then what ever does not flow through the injector between the time it pops open until the time it pops shut winds up back in the tank or at the filter.
I think perhaps this is what you are talking about. there will always be a return fuel line somewhere in the system that recirculates some of the fuel. My dodge truck pumps it back to the tank. My Dexta pumps it back to the fuel filter.
From your description I could be way off base but perhaps this will help