Firstly, I am never specifically aiming for a "grease landing" - much more important to be at the right speed and touchdown at the selected point, etc. but obviously the landing should not be "heavy".
A theory was once put forward that when the conditions were ideal we tend to use the left hand side of the brain (the logical, reasoning side) more than the right hand side (the creative artistic side).
When the conditions are far from ideal and you are working hard you are making more use of the right hand side brain and, perhaps, trusting more to instinct and feel and (surprise, surprise) end up with a smoother touchdown!!
I am no psychologist but this phenonema was covered in detail the Log several years ago.