Why would the reservoir be full and the radiator low on fluid?

Is the small return hose from the reservoir to the top of your radiator plugged or punctured?

I'd say the radiator cap is faulty. When the cap is sealed, as the engine cools it forms a vacuum that pulls the coolant back from the overflow bottle

I'd say, more generally, that in normal operation, as the engine cools it forms a vacuum that pulls the coolant back from the overflow bottle. If there are any air leaks into that system (e.g., the radiator cap leaks, one of the hose connections between the reservoir (aka overflow bottle) and radiator leaks, or the hose(s) itself leaks) the vacuum is lost and the coolant in the reservoir won't get sucked back into the radiator. I've also read that air pockets in the radiator can cause problems, and it makes sense to me that this could be one of them. The vacuum formed by the cooling would reduce the size of the air pockets, but perhaps (depends on their size) not enough to create sufficient vacuum to suck the coolant from the reservoir to the radiator. And if the hose is plugged, no coolant will flow unless the vacuum is sufficient to break the plug (which isn't the case; if it was, you wouldn't know it had happened, because the problem would have gone away).