I don't see how a buffer, or capacitor, or battery (without voltage divider) are any different from each other.
They all provide a solid point for the A/D to sample from. Either drawing or dumping some current.
What is important is that there is no ground difference between the three above sources and analogue ground.
Maybe the tests were flawed there.
A capacitor ofcourse changes ratiometric sampling into voltage sampling.
Something to watch out for if the cap is too big or Aref is too dirty.
Leo..