You're right, RP, there is such a set of rules based on seniority. 76 Squadron is 114th or 115th in the seniority listings (depending upon whether or not 360 was around long enough to overtake it). There are an awful lot of units with greater seniority than 76.
However.... the rules for reserve units are (or were) slightly different, and the issue of seniority has never been observed strictly here. If it had been, then the Tornado OCU would be 45(R) and what used to be MEFTS would be 15(R) - 45 is senior to 15 and shouldn't (under the seniority principle) have been renumbered post options for change, whatever Malcolm Rifkind had said about the disbanded RAFG Tornado units carrying on, albeit with different aeroplanes.
For instance, when the F-4 OCU was formed, it was originally planned to have the reserve identity of 68 Squadron, but Air Support Command requested that the air staff amend this to 64 Squadron. Not because 64 was more senior - which it is - but because ASC would use the F-4 in the FBSA role, and 64 had been used in the ground attack role in both world wars, when 68 Sqn had not. ACAS approved the switch.
For 76, it seems that the local connection, plus the famous VC (VC awards were also a factor in deciding between numberplates in the 1950s, BTW) have been used in a similar way here to that decision over the F-4 OCU, although this one is historically more accurate (68 Squadron had been a ground attack unit as well, albeit only in WW1). This also means that when Gordon Brown realises that the RAF needs more SH and AT [insert hollow laughter] the more senior numberplates can be resurrected to fly these airframes...