When the engine is fully up to temperature the coolant has expanded. If it was full to start with there's a greater volume of coolant than there is room for it in the colling system, so some is vented off. By the time it's cooled down enough for it to be safe to remove the cap it's contracted again, so the system no longer looks full. It's normal.
So it is really okay thanks then, I finnaly able to relieve a bit