Well I suppose it depends on the person. I know sometimes I pretend not to know a person's name or who they are because I don't want to come off as creepy for knowing their name when I haven't talked to them before. But that's usually the only time I've ever pretended not to know someone. If I've talked to them before, then it's silly to pretend that I don't know them. Other people may have other reasons that probably go along the lines of being socially awkward, actually forgetful, or just plain inconsiderate and rude.