Why did my best friend lie to my face?

I’ve posted about this previously, but one time, I googled my friend out of boredom, and found that her birthday was different than what she’d been telling me. She told me her birthday was on August 3rd, 1998, but everything I’d found (court records, voter registration) showed her birthday to be on October 10, 1998. I confronted her about it and she said that that wasn’t true. Well, we aren’t speaking right now due to different reasons, and I got bored again and decided to do more digging. (Which I know probably isn’t a good thing.) I found her birth record that has her parents name’s on it. Sure enough, it says October 10,1998. Why would she lie to my face about it? It’s not like I haven’t been celebrating her birthday on August 3rd. If she didn’t want me to celebrate her birthday or something, I would’ve understood. But why lie about it? My feelings are really hurt and I don’t know if I should just let it go or confront her about lying.