Going into nerd mode … there are some examples of going out from z2 where the default is via z1 and there is an avoiding z1 fare: Canonbury to Romford is a good example, where the default is Windrush to Whitechapel via z1 then EL. The faster, more direct route is Mildmay to Stratford then GA or EL requires a touch on the pink reader and is shown as avoiding z1.