People have all kinds of motivations for not filling right up.
- cannot carry more than e.g. 2 people
- the owner requires the plane to be returned nearly empty (esp. true for AOC/charter schools which may get a "heavy load" turning up the next day)
- there is friction within the group so nobody wants to put too much fuel in (ever been to university, shared a house with some others, and found the larder always empty
?)
- a lot of groups operate questionable-incentive policies for duty drawback reclaim, allowing e.g. the one member going abroad to pocket the
whole drawback, so you get e.g. a lot of trips from Lydd to LTQ usually by one member who is then being subsidised by the others; if you "play this right" you can wangle it (with a big enough tank) so the duty drawback alone funds a trip to LTQ from anywhere from Bournemouth to Lydd
(I had an instructor-renter do that; instructors know all the tricks)
- cost sharing schemes where the passenger doesn't want to pay for more fuel than necessary
- a lack of understanding of just how small the cruise speed hit is between full and half full. Aircraft performance is poorly understood on the GA scene, especially to this level of detail. The effect on IAS/TAS according to weight is not documented in most GA flight manuals.
etc...
But unless I am missing something, this ditching was caused by a misunderstanding of the finances, because they could have departed from the UK, with a full tank, flown direct anywhere in France, and reclaimed the whole tank as duty drawback - of the order of £100. That beats the cost of a landing + refuel in the CI anytime.