Again, just an educated guess, when the gearbox is hot, this is it's normal operating temperature, so it follows that when it is in this state is when it needs sufficient oil to protect it and dissipate heat, it doesn't really matter what the oil level is at when it's cooled down sitting on the driveway as it's not doing anything.
Always checking at the operating temperature as described also give a set reference point every time, eliminating potential discrepancies like different ambient temps changing the rate of cooling and so affecting the amount of expansion and contraction of components and oil itself.
Someone more knowledgeable may be able to correct me if I'm wrong as i'm no mechanic, still picking things up as I go along (every day's a school day!)