It can be difficult for people to admit they are wrong. They may be worried about being seen as a failure, or not good enough. It can be especially hard for a parent with their kids as they think they should be the role model of getting things right.
If you think about it, admitting you're wrong to someone else is pretty scary. It means acknowledging that you didn't think something through as clearly as you could have, or jumped to a wrong conclusion. Especially with your child, someone can worry that admitting they are wrong sends the message that the parent doesn't need to be listened to, after all, even they admit that they don't always get it right. Sometimes people just don't admit they are wrong out of fear of what the other person might say or think, or even due to low self esteem. Even though it would be lovely if mum owned her mistakes, please forgive her for not doing it as you'd hope she would. She's your mum, the only one you'll ever get, and she probably loves you more than anyone you know as well. :-)