The time that appears on the Web when you make your reservation is estimated, the duration of the transfer varies according to traffic, possible works, festivities, road cuts etc…
Remember that if you have to check your luggage, you must be at least at the airport 2 hours before the departure time of your flight.
As a tip, add 10 to 20 minutes more to the estimated time that appears on the web and add 2 hours (or the time you want to arrive in advance), so that they are at the airport, port or station with enough time.