I would say it has to do with three things: air pressure, air resistance and motion. A known example of this is called g-force.
If you use hooded car, the interior creates a "vacuum" and when you move the car at a steady speed with only minimal/slight accelerating/braking or sideway motion, you usually sense very minimal changes with your body if none at all, which means the vacuum little by little is adjusting to the motion that is in the outside world.
This means you can even jump inside the car without smashing into the back of the car or throw a paper aeroplane inside the car without it piercing thru your head immidiately, as there is no additional air resistance caused by the air that is passing by the car at 80km/h compared to what you would face when you use a convertible car or for example open the door of the car, that causes a sucking motion torwards the door because of the air pressure difference.
If you drive a hotrod racing car where in the beginning you try to accelerate as fast as a nerd can wank to a .jpg, you are pretty much glued to the seat because of the g-forces. Even if you brake a little harder without crashing into anything and you are standing, you might lose your footing in the bus if you win at failing, so that means that there is too sudden change in the motion between the inside and outside motion.
or something like that