You're not accounting for the compounding effect. U and DGAZ both track the percentage changes in NG, not the dollar value, and they compound daily. Like if you had a stock at $100, and it went up 50% in one day, it would be worth $150. Same $100 stock goes up 10% a day for 5 days and it's worth $161. The same goes for when it drops. Also you have to factor in the decay, and the fact that there was a rollover while in backwardation, and U/DGAZ changed contract months as well. Kinda complicated I know...