Which I guess is ultimately a result of the prohibition on fare-setting TOCs creating their own "TOC ONLY" walk-up fares. Relaxing this restriction seems to have worked well enough at Milton Keynes - perhaps it's time for it to be removed nationwide...
Has it really? The fares are within about 50p of each other and can't really reduce any further. All it does is cause confusion and limit choice. It's be easier for everyone if they just set the interavailable fare to the average of (income from) all the fares and competed for actual usage based on quality, frequency etc, to alter the ORCATS distribution.
By the way, they haven't "relaxed the restriction" at all, it's an anomaly caused by a change to the fare-setting TOC. MKC-London used to be a VTWC-set fare, it switched some time ago to being LM-set (possibly even in Silverlink days) as commuter services have progressively increased in frequency and speed over time. LM (SS) were allowed to keep any TOC specific fares they already had but not add any new ones, as is always the case when this happens. VT of course, as no longer the lead operator, can set theirs as they wish.
Here's an idea.
When we eventually move to true e-ticketing, which has to happen soon enough, ORCATS could be replaced by revenue distribution based on actual usage. So if the TOC grips the ticket, they get the revenue. If nobody grips it, the revenue is split based on usage of tickets which are gripped. So there's an incentive for TOCs to do proper on-board checks, too.