I have been reading a lot of mylifes posts and she sounds a lot like my ex.
Lets say you were at my house and i had one cookie left in my cookie jar when you arrived. And you were hungry and wanted a snack. I came in the kitchen just after you and wanted a cookie also. it is gone because you had obviously beaten me to it. I confront you and ask you if you took the last cookie. you deny it. WHY? knowing it does not matter and you know you did and i know you did, what is the point?
My ex did not ever get into it over a cookie but i was trying to make it simple. i have caught her lying about insignificant things. And my question to her always was why not tell the truth, it does not matter, it is not that big of a deal. much like the cookie incident above.