Why do parents seclude kids?

This may seem petty but I've been struggling with this for the past 4 years. I'm the mom of a 10 year old girl, Nicky is her name in this post, who seems to be pretty happy. She is an only child and I've tried to socialize her as much as possible growing up. It's been tough but I noticed that moms at her school have been participating in "social engineering" and exclude her often. I don't know why, she's social, gets good grades, teachers like her (never complained), and parents OUTSIDE of her school tell me she is so well behaved. She's at the age where she's starting to notice this exclusion and has been brushing it off. So she recently had a birthday party and invited about 15 kids that all came. They had a great time laughing a playing. Today, 2 weeks later, we get a call from one of the parents asking to borrow something and the dad says, "Oh by the way, it's Charlene's birthday and Nicky can come if she wants." Ok so since you need to borrow something, Nicky is now invited. This has happened since we moved here 4 years ago. We live in Oregon, in a supposedly very liberal city that is suppose to be pretty inclusive. Am I missing something here? I'd appreciate any advice. Please no rude comments, I know that's what some people spend most of their time doing on these forums but I just want some solid advice. She's starting middle school soon and I would like tips on how to help her have a healthy, enjoyable school year. Thanks! via /r/Parenting http://ift.tt/2vBPTFq