This has been a lifelong wonder of mine!
I have autism, which may explain why I don't get this.
How do you deal with someone (say a colleague, friend, or family member) who is always nice to you, but you know for a fact is horrible to one or two people?
I have been on both ends of this type of person. The problem is, when I have asked someone why they're mean to someone else, they have often turned on me, so I don't tend to ask now!
I just don't get why some nasty people have so many friends and such good social lives?!?