1, tail fan is acting in clean air is working less therefore heli is more parallel to the earth due to less tilting of the main disc to fight tail fan counter flow. Less effort needed to keep heli in hold.
2, clean air under the main disc usd so the flow is not disrupted and uneven in the flow path.
3, coning, blade tips are closer to the helis center of g fulcrum/intersection point when the tips are loaded toward it and cone negatively in the main chassis rsu position.
4, gyroscopic forces wanting to propel the disc in ways that increase instability while rsu and actually help in usd position.
5, tail rotor in rsu position is fighting down wash from the main rotor and fights harder to maintain hold while the heli must lean over to fight back, turbulence between the two fans disrupt downward flow as does the main body/chassis, tail fan in rsu is less effective.
6, the rotating mass of motor also has a gyroscopic effect on how the heli moves and it wants to rotate about one axis usd rather than tilt and move sideways while rsu when the motor is underneath the main disc, (take a motor spool it up in your hand held tightly on a mount or with screws in the mounting holes and try to make it follow a helis movements).
7, less disc sideways travel used to keep corrections and balance.
Draw a pendulum and place the disc at the top as in rsu. If the disc has to move twice as far to any horizontal direction to pass the chassis point of reference as it is off center (imagine the disc is now twice as far to one side after the heli offsets from true vertical) while in usd the disc does not have to travel very much to achieve catching the heli before it slides away (like a game of catch up).
So there, have fun, flame suit on, your turn, lol, (just thinking out loud).