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I did not add this vespene manually nor did I something else to the fleets after sending them.
So I have to assume the vespene calculation for the flights added too much vespene or that it added not enough vespene and my recycling fleets returned at 90% instead of 100% speed.
Consumption depends of unit speed: bigger speed - bigge consumption. When you sent fleet you had level of Evolution of muscles N, when these fleet returned you had level of Evolution of muscles N + 1. Not a bug.