This bug has been reported many many times and even by members of the staff. My best, and as unofficial as can be, guess would be that something went wrong with either porting the forum user tables from YaBB SE to SMF or with installing mambo to handle the userbase.
It's some issue with the username not being the same as the user's nickname when it should. YaBB SE allowed the user to change his display name ie. nickname to be different from the actual username ie. loginname. Now, if I remember correctly this first was the case with SMF too, but I don't know if it is anymore. Also, who knows which of these names mambo uses and which the SMF IM system uses.