Is there an alternate meaning to constant lieing?

I've been married for almost 9 years now. My wife has had an issue with lieing since our first year of marrage. She mostly lies about petty things like, "I only spent $30" when it was actually $100. "I swear I paid that Bill" and really dumb things like that. She doesn't normally lie about important things and usually after 5 minutes of checking the account and making a few phone calls I get to the truth. The lies are more of an irritation than anything else but after a while it starts to eat away at the trust we are supposed to share, and when I get really irritated with all the lies my brain (who has a mind of his own) comes up with thoughts of "She doesn't respect you" and "It's cause she secretly hates you". Those thoughts only happen after a really bad day though. Now like i said we have been married for almost 9 years and we have 2 kids together. Strangely my kids even know that lieing is wrong and it blows my mind when she will catch them in a lie and spazzes. Now, other than the lieing we have a great relationship. She was and is still my best friend. I have never belittled her or abused her. I have never given her a reason to fear me or be scared of telling the truth. In the case of the bills I have told her countless times, "look, if you didn't pay it just let me know so i can do it." Though it never works. Is there something i can do different, or is there some way I can get through to her?