Yep that's right, I'm number one and whoever registered after me has number 2 and so on. With every registration, this unique ID auto increases with one and defines who you are. When you're logged in, the site doesn't do anything with your username, it just stores your user ID. And whenever you post a forum message, the forum doesn't do anything with your username but stores your ID along with your posted message and the topic ID.
In that way, usernames can technically be modified without making other adjustments. Plus, it's faster to work with numbers than with names.
I'm on the 351 position.. And the 2nd position of what-you-know...
I'm quite old in the Forum, and I'm proud of that.
PS. When I joined U2start and I began to participate on the Forum, I saw i was user 351, and I wrote a big black "351" in my Phyolosophy notebook.
By the way this notebook is the one where I did most of my setlists...