Well, it could be a mistake by the Motor Vehicle Registration authorities as in my case, Motor Vehicle registration guys gave my father a new number for his car. We had this rolla 86 re-registered with Islamabad with IDG series in 1994 after transfer from Karachi. In 1999, at the time of paying token tax, my father was told that this number has been mistakenly issued to us and therefore we need to get a new no. with series IDJ. I saw a Daewoo Racer a few times with our old registration no. but I still wonder why it took them 7 years to find out the mistake. Now my father's car had 3 different registration numbers with single owner. Strange things happen in the land of the pure.
Also, sometimes various agencies (not to be named here for obvious reasons) do get those registrations nos being already used by the general public or so I heard.