[e12e]

My guess: Less water means less heat energy, meaning you don't "overheat" the coffee as badly. It tends to bring out the bitter taste. You find similar things with (proper) tea -- I suppose most infusions.

If you only have one french press it might be difficult to experiment; but like the top comment says; if you want to find out why, getting a thermometer might help (or a timer -- boil water and leave it to cool for x minutes before pouring -- would be somewhat harder to replicate with different ambient temperatures and different vessel, though).