It’s simple. Atheists form groups because they want to meet other people who they can discuss their opinions with. Many atheists in addition have different ideas with what to do with their atheism and like to spend time with like minded people. It’s the same with football I suppose. There are people who support Manchester City and they buy season tickets and sit in the sleet singing Blue Moon, and there are those who go to a handful of matches and watch down the pub. Now the “true” fan is neither, they are both fans, it’s just that they have different ideas to what they should do with their support.
Skepticism lead to my atheism so I use my education to provide medical advice to anti-medical skeptics in addition to campaigning (and indeed physically working against) religious issues that affect healthcare. We all have different things we are involved with. And that goes with most groups. Most religious people are ad diverse bunch too.