Human observation: The purpose of unnecessary "white lies"

The scene: Person A is a good friend of person B. A stays in contact with a group of old friends after she moved out of town. Person C works in the same place with B, she has no close relationship with neither A nor B, as she has a different social circle. Later on, when A revisited her friends in town, she ran into C and invited her out for a drink.

During the conversation, A causally asked if B has ever socialized with B. Since they used to work in the same department before B transferred to the other one. C expressed her surprise and said she thought B has quitted and moved away.

In fact, B and C almost run into each other everyday. Just the day before this conversation, they saw each other in the hallway. B and C are not friends, neither are they enemies. Sometimes they nod to each other, while most of times they just go on their own ways in a hurry.

The very next day after this conversation occurred, C encountered B again at the same place in the hallway. C greeted B with a smile.

The observation: It seems strange at the first sight that C lied, since it's completely unnecessary. She could simply tell the truth which does no harm to anyone.

The curious phenomenon is: B answered A's question with no hesitations. She spoke in a natural and convincing manner that could even make people question the realty, if she actually never recognize a person she sees daily.

Of course it's not a big deal. I'm just curious why some people "naturally" lie on unnecessary things. Do they really believe in what they say so they have no negative feelings about it?

The other person I know who lies on a daily basis is my Mom. Though I know her behaviors relate to her cultural background and brought up. She's really a master of story-telling, this capacity brings her a great success (in contrast, my Dad, insisted on living with honest, failed both socially and financially). She does not only convince others but also herself on some levels, so I grow up questioning my perception of the realty constantly. It's confusing when an "authority" firmly rejects your first-hand knowledge of the realty and tells you otherwise, especially when you were a child. It's somehow an epistemological problem too: if one's sense experiences are unreliable, then what could constitute the Justified True Belief ?