I get the same effect. I doubt it's because of any precision requirement in the software. The quality setting specifies the image resolution used during dense matching: Ultra = original pixel, High = 1/4 original, Medium = 1/16 original, etc. At a fine scale features may appear very different in two images, but when you coarsen the image those features are likely to appear more similar in the separate images because you've reduced the local variability.
The result of using a lower quality setting on rough surfaces like tree canopies is a lower point density (because there are fewer pixels to match), but you often a greater coverage of points (because it's easier to find matches). Higher quality setting are likely to yield more accurate point locations which can be important if you want to get accurate height measurements when working with coarse imagery. For UAV applications where the approximate ground sampling distance is less than 10cm this is probably less important, but when working with imagery that has a gsd >30cm I've found my height measurements for trees are significantly more accurate with Ultra quality.
What still baffles me is that I often get better coverage with Aggressive filtering. I wish someone would explain the filtering settings to me more clearly.