Epping was only added to the TfL zone system recently (1998ish) -
previously, the stations on the Epping branch of the Central Line outside of Greater London
behaved like the Met stations beyond Moor Park do now. The idea is to simplify life for Tube
travellers (and for TfL ticketing!). The stations around Hainault, many of which are also in
Essex, were in the zone stysem from the start to simplify life for passengers going round the
loop.

All of the stations on the Hainult loop are within walking distance of the Greater
London boundary. The main reason for there inclusion was probably so a journey
from Woodford to Hainault (both in London) would not be charged excessively
because of passing briefly into Essex.