On 20 Apr 2007 00:37:37 -0700, Mr Thant wrote:

I was checking fares on the NR journey planner and noticed cheap day
returns from South Tottenham (Z3) to Upper Holloway and Gospel Oak
(both Z2) cost £2.40 and £2.70, when under the zone rules they should
both be £2.80.

Digging further, tickets to places further around the NLL seem to
increase gradually with distance (eg South Acton is £4.50), even
though it's still all Z2-3 and should all be the same price.

What am I missing?

The National Fares Manual includes a 3-letter code next to each fare
to indicate which company sets that fare. Most NR zonal fares are
shown as being set by "DBA" (not sure who that is). But the NLL ones
you quote are shown as being set by STS (Silverlink). So it does
indeed seem to be an exception.