This can happen if you want a one way ticket. If the part you want is medium, booking a one way award ticket will price as if it were medium in both directions. However, if you can find a throw-away return in low, it will price as the average of medium and low, thereby saving you some miles (but probably costing a bit more in taxes and fees). IF there's married segment logic being used for award availability, booking more segments might get you want you want in pure low.