Most internet folk seem to peg a bunch of numbers next to a generic lowercase name/title, like "demonhunter", "cyberzero" or "frontlumps". 607 decided to skip the formalities.
I once did, but he said he didn't want to explain.
Or, like what @Wrangler of Lake Aspargus somewhat jokingly said, it may be just a generic, possibly even randomly generated number.
When I first joined Wario Forums as my first online community ever, I considered the name "Zeta9947" (which I recently have made a cleverly hidden reference to) but ultimately decided that the still extremely generic "Just a Wario Fan" expressed a bit more personality.
I may be wrong, but I think I was the first one here to abbreviate it into "J.A.W.F".
"Just a Wario Fan" was such a mouthful to type every time, we needed a good shorthand version (the same can be said for my obnoxiously long username).