Oh, it's because lying well is great creative fun, and it doesn't get much better than lying successfully to someone who thinks they know you well. Everybody lies to everyone all the time, about everything: that's what everything in life is all about.
Oh, people say they want the truth, but really they want comforting lies. You don't see many people volunteering to investigate the only real truth in life, by putting their thumb on a rock and hitting it with a very big hammer.
Besides, she deserves it for all of the times she has left piles of shoes all over the stairs, waiting for an "accident" no doubt.