There are two additional factors that occur while riding the bicycle outside, which are not sufficiently simulated by a stationery bike in the laboratory. These include wind and road surface. Both demand higher cycle cadence to maintain efficient power output. The wind, because it's gusting, is partially offset by the gyroscopic effect of cycle cadence. Rough road surfaces result in a dampening effect.
Energy is lost through the legs.
The latter is partially offset by higher cycle cadence. As the wind increases, efficient power output increases with higher cycle cadence. Thus, riders must be selected carefully for similar power output characteristics. Power output varies with height and weight. Road speed can be the same even though power output is different.