Why do some people constantly change their stories?

My oldest daughter tends to say one thing with such utter certainty, then the next time I see her she says exactly the opposite, with such utter certainty.
For example, one day she made a comment that she sees ALL the comments I post on Facebook. She doesn’t miss a single one.
The next time I saw her she said she never even looks at Facebook and has no idea what people are posting.

One day she said she loves salt, especially gourmet salts. The next time she says she never uses salt because she doesn’t like it.

These are just a couple of examples, but she’s really consistent about changing her story. The result is, I take just about everything she says with a grain of salt ha ha. No pun intended….and to me that is sad.

But what would be the motivation behind someone doing something like that?