While I'm not an engineer or an MOD procurement chap even I know a bit about fleet management, as I assumed most aircrew do. You always buy more aeroplanes than you need to man the front line and training roles, to allow for losses during the life of the type, management of flying hours evenly across the fleet, aircraft being off line for long term maintainence, etc, etc, etc.....
A navy ship can spent 18 months to 2 years in refit. Therefore the navy has more ships than it needs to man the front line to allow for this fact. It is no different with aircraft. As for why so many Tucanos, another factor is probably that the numbers bought were to support the larger RAF of the mid 90's. Cut back on Sqns, pilots etc, less need for training/recruitment therefore less need for training aeroplanes. No hiden agenda and none of it is rocket science!