This is a hold over from how the site evolved. There was a very early comments system before the main forum was set up. When the forum was started the registrations were carried over (along with their start date) and at this time I believe arn created his official account as a poster.
I may be wrong with what I have said as I am going from memory from what I was told before as I wasn't around at that time.
arn and blakespot worked together to create macrumors and often work together on other projects toucharcade being their most recent venture.