The difference between European and US filler necks is, a European neck is deeper, from the top of the neck to the sealing surface. Accordingly, a US spec rad cap is shorter than a European rad cap. If you put a US cap on a European neck, it doesn't seat with the pressure the cap was designed for. That reduces the "release" pressure (or temperature) by a significant amount.
To answer your previous question, in an OEM cooling system, the shorter (pressure) tank requires a pressure rated rad cap and the taller (coolant recovery) tank uses an un-pressurized, vented cap.