Each matz only has 10 numbers each, so on a busy day, they would run out if they gave everyone a separate number. At leeming, it seems to be common for aircraft who are only requesting FIS in VFR to be given 0401, ie aircraft not under control of that radar, and the other 9 to be given to aircraft that need an individal squark, ie those requesting RIS/RAS or flying IFR
NB