I have personally found that my children lie to me for two main reasons. The first reason is that they are scared to tell the truth for fear of getting into trouble. No matter how many times I have explained to them that lying is wrong and it is better to tell the truth and face the consequences, than to get caught lying and face even worse consequences. It's human nature to lie.
The second reason tweens lie is that they just want to do their own thing and try to manipulate their way into getting what they want. They "think" they have lying as a power on their side to get what they want. Sometimes it works...sometimes it doesn't. It is in this case that hurting someone else may prevail over doing what is right and honest.
If you think about it, we as parents are supposed to be the best example for our kids. How often do we lie, and is there a difference between bad lies and lies that are acceptable? For example, I lied to my husband when I told him we were just going out to dinner for his birthday, but really had a room of 30 people waiting to surprise him. I've told my kids to tell someone I wasn't home when the phone rang. So, what do you about this?
I believe that a conversation is in order. Talk with your tween about lies and give examples of how lies can hurt someone else and hurt himself. Have boundaries for what lies are acceptable (i.e., to make someone else feel good) and which ones are not. Stick to your boundaries. Lies happen. They are annoying, but they won't go away. Having rules around lies is the best way to go.