2000 Mustang antifreeze leak?

I filled up on antifreeze about a month ago after my thermometer showed that the car was overheating. Recently I had the same issue and had to refill on antifreeze so I'm using there is like a pinhole leak somewhere. However, there is alot of antifreeze in the reservoir, it's almost full. My main question is, why is there alot of antifreeze in the reservoir, and not in the radiator itself?