It's a server load saving measure, I'd imagine. Instead of asking the server to generate tons of random numbers (since people /random and lot all the time), your client just sends a ping to the server and the server sends a number back based on when that ping is received. That time is just a measure of the Vanadiel Minute. Each VD Minute can be split 1000 ways. If the server receives your ping at the very start of a new VD minute, you'll roll a 999. If the server receives your ping at the end of a VD minute, you roll a 0. Everything in between is returned linearly.
There's slight variation based on internet connection speed (speed of light isn't instant, etc), since the number returned is based on when the server receives the ping rather than when the ping is sent, but that's very minor in most cases unless you're living in a country with fairly poor network infrastructure.
This was all figured out years ago. It's why you can also repeatedly generate the same number (or extremely close to it, variations due to network travel) if you set up a script to /random at set intervals corresponding to the length of a VD minute.
It takes a bit of skill to pull off, so I'm not really worried about people figuring it out. I rarely ever use this trick, it's generally not worth the effort. It's neat to know, though. I think it's fairly ingenious from an efficiency standpoint.