Galopping fuel consumption

My vamilly (rightly?) think I am a nutter because I religiously log  fuel quantity and distances and always fill the tank to the brim. I often do long runs, Annecy to Dunkirk for example. My Honda CRV has a pathetically small fuel tank which means refuelling once and more likely 3 times, fuel in the UK beinb excruciatingly expensive I like to arrive with  full tank. I have noticed that at each refuel stop there is an incremental increase in the consumption. This is confirmed both by the onboard computer and by my calculation though there is always a slight difference between the two. I am well aware that this seems impossible but it has been my experience with 2 cars. Long runs solo result in a stable consumption.