Actually the answer is very simple.
The forum software records all IP addresses used, and it is then very easy to link multiple IPs to one user, or even to discover other users using the same IP. There is no mystery here.
That's why (after only one post) I was able to connect Peroon's 2 nicks. There are also other users here with more than one nick, but as long as they don't cause a problem then it's ok. There are even some users here with multiple nicks which they think I don't know about!
This is
not the only method available to Admin. As they say, there's more than one way to skin a cat.
Big brother is watching...