The thermostat simply regulates the temperature by opening and closing to keep the coolant at 90°c. It doesn't affect how much heat the coolant absorbs from the engine. So if the engine gets hot then the coolant gets hot.
The stat should be closed initially so that the engine warms up to 90°c, and will then open/close as required to keep it at that temperature. If it's stuck open then the 90°c will still be achieved (albeit a longer time to get there) and then the cooling fans/water pump will keep it at 90°c and stop it overheating. When you're driving the coolant is still circulating round the system and radiator, and the cool air coming in through the grille will cool it down. This is when the stat should be closed to stop it cooling, but your faulty stat is stuck open so doesn't regulate the temperature correctly.
