Traveline data for England comes from councils direct and so it would be my guess the local council has set it up as that timetable.
Not necessarily.
The regional Traveline structure that was set up in the early 00's is no more.
Government theory is that bus operators put all their timetable data in to BODS (Bus Open Data Service) and that local transport authorities don't need to get involved other than maintaining the NAPTAN (database of bus stops) data, so some councils don't do anything more than that (and there are some councils where it's fairly hard to get them to do anything with their NAPTAN data but that's another story.) The quality of some bus operators' BODS data, and how out of date some of it is, shows this maybe wasn't such a good idea...
Some areas still have a regional Traveline and councils who take an interest in this - Traveline South West is still a thing, and some authorities outside the South West have joined them following Traveline South East being disbanded. I don't know the Sussex patch well enough to know what the relevant council's approach to it all is.
Traveline also gets data from the rail industry which does not go through councils, there's a national coach services data set (national as in country wide, not national as in National Express that is, although there are a few operators whose coach services aren't in it) that's handled at national level not via local councils, and I'm not quite sure where data for light rail and odd things like the Gatwick Airport thing (whatever it is) is supposed to come from - I have been involved at various stages of this, but never anywhere with light rail or anything like this.
Going back to the specific, a possible difficulty with the thing at Gatwick is that it's not a registered local bus service, so it doesn't have to run to a set timetable, and there's no requirement for the operator to give notice / information to the local transport authority if the timetable changes. I've not travelled through Gatwick for 20+ years, so don't know how it operates in practice. If it's more of an 'every few minutes, depending on demand' arrangement, then that's very difficult to put in to a timetable form that a journey planner can cope with, so it may be that someone has made a conscious decision to inflate the headway / running time a bit to allow for 'worst case' so that intending passengers don't try to make too tight a connection on to a train and then complain when they miss it. (Last time I looked for info on the similar thing at Luton, National Rail Enquiries showed it running every 1 minute, which is possibly going too far the other way.)
Alternatively, it's possible that the Gatwick thing has fallen in to a hole where everyone thinks it's someone else's responsibility.
I presume this is about the national Traveline site (traveline.info) rather than any of the regional sites? If so, there is a web form for contact / feedback
here, so may be worth getting in touch.
bustimes.org
