My assumption is that the water level represents the center of an oscillating waveform, so it is "top to low" in the sense that it goes from the top of the wave

My assumption is that the water level represents the center of an oscillating waveform, so it is "top to low" in the sense that it goes from the top of the wave to the "low" representing the normal water level, and it is "middle to top" in the sense that it goes from the (middle) inflection point to the maxima. For the former, note that oscillations may cause a lower amplitude less than the normal water level, but it may look more shallow due to the different diffusion of water relative to air.