Suppose you have a level less than single-screen, so there are grey inactive areas around the play area. Now have two lemmings close to the edge, so you need your cursor to overlap A but not B, in order to assign to A. Sometimes the cursor will have to move off the play area for this. If it does, NL will refuse to let the cursor "see" the lemming and assign it the skill, even though it's within normal overlapping distance.