there is also the posibility that that your radiator has a crack in it that would not allow the suction of the fluid in the over flow bottle to be sucked back into the radiator due to the fact that the suction that is in the radiator would be sucking air in coz there would be less resistance than pulling the coolant back through the hose.

but, in the sutuation that i've just suggested, the overflow bottle usually gets full & doesn't go back down to half level.. it would stay at the full point.