In the past Zap Numbers were a local affair, approached differently by different units and sub-units. At one time, when the only game in town was Northern Ireland, they were known as NIPIN (either Northern Ireland or National Insurance Personal Identification Number, depending on whom you believe).
The exact format often conformed to local arrangements, normally at Coy/Sqn level. For instance when Chickenpunk first went out to NI he was Zap 04 in the company list. Other companies in the battalion did it entirely differently, which didn't matter because they weren't on the same net.
Since then, it has become necessary to adopt a standard across the British Army to avoid confusion when units and sub-units are regrouped or temporarily come onto another net, either for movement control purposes or because they have lost comms on their own net. Having a standard also means you don't have to keep changing your Zap Number every time you change company or go on another tour.