If you find your workouts intervals in FulGaz are incorrect, please ensure you have selected STEADY mode, not REACTIVE or CHALLENGE modes. Riding in STEADY mode keeps the video speed at the correct speed for your workout, thus keeping the interval length correct.
Click on the settings "cog" in the top right corner of the main ride screen.
Under Ride Modes, select STEADY from the three modes.
Riding in REACTIVE mode will change the video speed to match your power output and will affect the length of any intervals.