Not sure it is as simple as that - geographical stuff comes into play too. Kent borders only London and Sussex (tiny bit of surrey too), and a large part of Kent (Thanet) is too long to commute to if you live outside of Kent. Realistically you can only commute if you live in London, but why would anyone want to do that, especially given that London has much higher levels of vacancies than Kent (1,493 versus 194, according to Guardian teaching website).
If you look at same guardian website there are precisely 5 mgmt-level vacancies in teaching, similar to other counties. Now am sure there are different roles advertised on different sites, but I doubt the numbers will stack up particularly against Kent.
Seems like simply good tactics on the part of the council to cast their net wide in search of good talent.