Kettering was never served by London suburban services and was outside the London & South East fares area, so when BR moved from Regions to Sectors it became an InterCity station instead of Network SouthEast. It's current fares structure for journeys to/from London is a consequence of that.
Kettering may not be that much further away from London than Northampton is, but the quirks of railway geography mean that the former IS a long-distance journey from London whilst the latter isn't
Ok, so the explanation - more understandable than wrong claims of any notably improved service - is that Kettering happened to be in the wrong place at the wrong time, served by the wrong type of train, at the time of privatisation.
At a glance, that explanation (not meaning it's OK) holds up in comparing to Peterborough. That is further north, but was lucky enough to have both IC and suburban services. Today that translates into £65 anytime return for a very reasonable 1 hour 15 minute journey, versus £118 anytime return for an hour.
Throwing Corby into the mix, which only got trains relatively recently, the only continued "justification" for including Kettering (and in turn it) into an IC inflated price grouping appears to be the different rolling stock.
When Corby, Kettering and Wellingborough move into Thameslink in all but name (and lacking direct cross-London connectivity), being served by only suburban EMUs, perhaps their fares can be revisited then?!... Including noting that Kettering's walk up off-peak fare of £79 is more expensive than Peterborough's peak Thameslink return. And only £13 cheaper than Manchester's walk-up off-peak IC return fare.
Given a train company tried to run the service with suburban DMUs once, and still increased the fares, I don't hold out much hope.
Taking out any notion that Kettering has been treated exceptionally, it in any case opens a wider question as to how it can be justified for IC routes, this one in particular, to have such runaway fares. I definitely can't see anything to defend the prices.