The split at Hove works because a few years ago First Capital Connect introduced super off peak days fares on their network at weekends which are significantly cheaper. Although FCC didn't serve Hove its fares are clustered with Brighton.
I thought it might be something to do with the Thameslink route.
The other splits utilise off peak day fares which are available on shorter flows whereas only period returns are generally valid on longer distances.
They don't expect anyone to be going from Worthing to Nottingham & coming back the same day
One thing I forgot to mention is that your train must stop at Bedford as well as Kettering. This shouldn't be an issue as I believe all the Kettering trains at the weekend also stop at Bedford but do check before boarding.
They do - I have been to Bedford & used a Nottingham-bound EMT to get there from London - much better than Thameslink
And these fares are all 'walk up', available on the day of travel at the same price with none of the inflexibility of Advance fares.
I don't have to get off at the split points though do I - but I do have the freedom to if I want to, say, get the Corby train & get off in Kettering or Bedford for a bit?