The sign above these two holes in a canteen says “please dispose of food waste separately from other waste” – but does not specify which hole to use for which. That is done by the debris around one of the holes and the relative cleanliness of the other.
Stigmergy at work.
Petafloptimism 
Might it merely imply that right-handed people are messier? Being right-handed, I lean that way.
An appropriate experimental response might involve cleaning them off and placing scraps around the left one….
It would also be interesting to observe the people whether they would notice that they are being experimented on – obviously you could not tell them beforehand :-).