A few entitled a**holes appoint themselves as the authority of a "list". They stack the names with all of their friends first, and then everyone else afterwards. Then they "change" the "check-in times" without notice to ensure only they and their friends get top spots on the "list". They commiserate with the venue security to validate their purpose, as the venue security gives zero effs about patrolling who is in line fairly. And then when you post the truth on a forum such as this, they gaslight and b**ch about it. Good luck!
I don't know which show(s) you attended, but I can say that this is not always the case. When I went to the East Lansing show in 2011, the experience was like this:
We showed up at the line and someone in front of us asked if we had gotten our line number yet. We hadn't, so they handed us a sharpie, told us what numbers the people right in front of us were, and we were the numbers after this. We waited in line according to what number we were and when show time came, we got in easily and got a great place close to the outer ring (and within 10 feet of the band as they walked by). We had zero issues with anyone in the line or the line leader.
I've heard of other lines where whoever the first one to show up starts the line. Sometimes they bring a notebook and write down people in there; other times, it's just the sharpie number. It all depends on where you are and who's doing it.
See here for a good summary of what GA lines tend to be like around the world: You are not allowed to view links.