However, there is one study that contradicts those:
Differences between sprint tests under laboratory and actual cycling conditions
In conclusion, the results of this study indicate that POpeak, Fo, and time to obtain POpeak were different between laboratory and actual cycling conditions. To perform a valid estimation of the POpeak in the seated and standing positions in laboratory it would be necessary to use the personal bicycle of the cyclist and a set-up that permits lateral oscillations. However, the better sprint tests seem to be those performed in actual cycling.
So, basically, the lab bikes were awkward to sprint on and that may account for the lower numbers in the lab than on biies.