The Buggatti does not have power settings. It does have a top speed key but this only effects aerodynamics mostly drag and lift. These effects are more pronounced as speed increases.
Most likely the Bugatti is plagued by its on board traction control system in the wet. If its anything like any other VW traction control algorithm its probably at 50-75% power at the slightest hint of wheel-spin.
That being said. it still appears to win the second race and then hit the brakes before the GTR goes by.