ahh i see what your talking about now... the way the bottle fills it traps air at the top of the tank as the neck of the fill ube sits into it a bit Therefore when you fill it you get a bit of air trapped.
then you move the car the water moves and moves the air which then escapes up the tube
dont believe me fill it up as normal then get a mate to give the car a quick shake and you will see the air bubbling up