Willard, why the use of 'tranche'? Simple, it is fashionable and, in the RAF it is associated with modern, forward-looking and punchy, CDS when these decisions were made was Jock Stirrup.
From Wiki:
In other fields, notably aviation, the noun tranche is used to denote a block of units sharing a common standard. The
Eurofighter Typhoon which is used by a number of European air forces, was built to a number of successive tranches.
Vim, I'm not sure 'cunning' applies here, that would be giving the leadership far too much credit.