Going back when the OGs were new and each assigned depots in orderly groups. Now its more random like the RTSs, seems like when a bus fleet gets older it gets scattered around.
Newer buses like the NF and the NGs are also scattered around. It's not limited to just the old buses. The only way you get a concentration of buses in one area would be the 3800-3959 NGs or the CNG O7s.