Poz. There could be many reasons why your numbers may not make sense.. If it is the Exact same wing with no Design changes.. The only things i would say would be different flying on that day, climbing harder, or higher. A few 100 rpm over the hour will change your figure a fair bit
Are these figures from one flight?or average over X hours.... Did you weight the fuel in and then run out or weigh out left over fuel?
I have noticed that different days will return different Eco figures with the same wing. The 250 is great with atmospheric changes unlike some other engines/carbs and can be tuned lean way past where other engines would go POP due to the great cooling, Doing this you do risk long term bearing failure..
Your figure are around the same figures my Mac fly 250 customers are reporting Circa 3lph flying sub 20m wings. My Viper 3 22m was 2lph @ 115kg all up, My speedster2 24m is 2.3-2.4lph and my Viper 4 18m is 2.35... Flying Faster with more agility when i want to play, and as an added bonus beating my old four stroke eco figures for around an extra 5kg added weight but with about 30kg's more thrust.. win win!!!