Bristlenose Pleco Cleaning Mystery Snail Shell

This is so cool and weird. We have two different plecos in the tank and about 4 dozen snails of 5 different types. Only this pleco is willing to clean snail shells and only the apple snails- none of the others. But we do have three of these snails and he does it with all three. So it seems very clear that somehow THIS pleco has made an arrangement with these PARTICULAR snails, ONLY. We are not the type of people that generally ascribe human traits to animals- particularly to small more primitive creatures, but we just can't figure this out any other way.