For outboundary Travelcards, my assumption is that there is an agreed
sum from each outboundary Travelcard (Day - whether Anytime or off-
peak - or season) which gets paid into the 'Travelcard pot' to cover
the Travelcard element of the ticket. I'd imagine this sum is rather
less than the price of an inboundary z1-6 Day Travelcard (let's ignore
seasons for the moment). However I find it hard to imagine that
different TOCs pay different amounts into the 'Travelcard pot' - i.e.
I reckon an outboundary Day Travelcard from Brighton, Cambridge,
Reading, Southend and Winchester all result in an identical payment
into the 'Travelcard pot' for the Travelcard element of the ticket.
(Bear in mind it's not TfL charging the TOCs, it's the TfL and the
TOCs collectively charging the TOCs.)

Therefore it comes down to what a TOC can extract from a punter -
albeit in the context of the rail fares regulatory regime, inc. the
fares baskets and RPI limits on fare increases and all that. Some TOCs
only charge a small premium for the Travelcard add-on - presumably
they think it's worthwhile doing so in order to attract punters to
travel with them (thinking here of off-peak Day Travelcards, which are
aimed at leisure pax), and perhaps they might actually end up netting
less than a CDR to London (?) - other TOCs, such as FCC, seem to
regard it as a way of milking punters. (The chunk of the ticket price
that doesn't go into the 'Travelcard pot' will go into the railway's
complex ORCATS system for allocating revenue amongst the appropriate
operators - from Cambridge, say, the predominant chunk of ticket sales
income for any permitted tickets to London and Travelcards to London
will go to FCC, who provide the fastest service with lots of
capacity.)

The 'Travelcard pot' is then divvied up by what I can only imagine is
a mindbendingly complicated formula as agreed by TfL and the TOCs
(through the ATOC London Schemes Council), based on all sorts of usage
data.

Which is a long way of saying that this is not a case of TfL deciding
they don't like FCC because they smell and charging them lots, whilst
being nice to say c2c because they bring cakes along. FCC or any other
TOC claiming its all TfL's fault would simply be talking nonsense
(though I wouldn't be remotely surprised if that's what you get from
the lower echelons of a TOCs customer service bods - though to be fair
it's a complex issue, and inevitably, they'll have been told f-all
about it, and so will just be following the line to take - though if
you come across the more senior TOC managers, don't let them brush you
off with the lame blame game).

I wonder whether these differentials are historic - maybe they go right back
to NSE days?