It may be something like your accent. I don't have an accent and neither does anybody else who was brought up in the same village as me. On the other hand, I bet somebody from Leeds could easily identify that I was brought up in the South East of England from the way I talk.
That's right - it depends entirely on your ability to perceive the full spectrum of religiosity. So someone may be a regular churchgoer but somehow see themselves as being in the middle ground of religiosity as there are others far more fervent in their church, but fail to recognise that just by being a regular church goer they are already right at one end of the religiosity spectrum as less than 10% of people are churchgoers.
And of course religious organisations can be all encompassing, effectively creating entire communities that people operate in largely only engaging with other people from that community. My wife is a good example - brought up in the RCC, I think she will freely admit that it wasn't until she was about 16 that she had any meaningful engagement with non catholics, such was the reach of the RCC community. So not only were her family (obviously catholic), but their lives, social lives etc revolved around the church. She went to a catholic school at a time when such schools basically only catered for the RCC community, so all her friends were also part of that community. And this community went further still - they'd never just get a plumber, but got a catholic plumber, effectively there was a sub-set of service providers etc also from the same RCC community that they always used.
If you were brought up in that community it is perhaps little wonder that you might lack a little perspective, coming to think that everyone goes to church, attends communion classes, is confirmed, goes to chuch-organised youth clubs, mid-night mass a big deal on Christmas eve etc etc because those are the people you see all around you - when of course that isn't the norm at all when you look at the full spectrum of religiosity.