We have to consider that all double deckers within the fleet aren't actually 'spare'. Some are allocated on a daily basis to certain services.
Even if they aren't officially allocated to the service, some depots seem to have problems allocating certain vehicles to certain services so they all end up out anyway (big flashy sign saying 'WASHINGTON' should be appearing in your minds, everyone!)
Those that aren't actually in service could be VOR. 3979 is just one example of that being so. Was it six Bishop Solars VOR someone mentioned the other day? If so, imagine all other vehicles Deptford have VOR... Deptford will literally have every single double decker out, I imagine. They don't have that many to begin with. I imagine a fair proportion of their "Northern" singles will be out too.
RE: Go North East