Why would someone lie so much?

So i have known this girl for almost 10 years, and if i had a best friend i guess it would be her. Shes an awesome person for the most part and we have a lot in common but she lies about pretty much everything, even stupid crap that no one really cares about. I know she lies because i have a really good memory and she tells me the same story over and over but always changes key elements. Like she was buying her house but her landlord was nervous about selling because she still owed money on it to the bank. Then a month later my friend tells me her landlord paid the house off 10 yrs ago so all rent is profit and thats why she wants to sell! Heres another one, she told me her car was paid off (i can do math and i knew she was lying anyway but i kept my mouth shut) 3 months later im at her house and there is a knock at the door and its the finance company coming to repo her "paid off" car. She will literally lie about what shes doing, to where she is going, to what she paid for a damn shirt! I dont get it. I have never competed with this girl before, she has her life and i have mine and i am in no way jealous of her life so why would she lie about everything? I called her on one of her lies a couple weeks ago and she just lied more to cover her lie! I dont get it, an idea why she would be like this?