NG up or down is based on the price at the 2:30 pm closing. Price at at time of stock exchange 4 pm closing will be different, but it is the 2:30 time that is official.
If you understood that the trading session ends at 2:30 pm you would understand how NG could be down but dgaz up.
Over simplified example: SET Time NG is at $6 and DGAZ trades at $3. For whatever reason NG sells off to $5 at 4:pm and DGAZ closes at $3.50. Next morning NG rallies to $5.75 and is actually down .25 from the "official" closing price of $6.00 but is up from the 4pm price of $5.75. DGAZ would actually be trading down at this point even though NG is considered to be up.
This what I think. Trading for Wed 2/19/14 was closed at 3 or 3:30 PM? Not sure of the time. The SET price for Wed was $6.149 for March and $4.950 for the April contract. NG then starts trading again after the set time and at 4:00 PM March was $6.081 and April was $4.869 and DGAZ was priced at the 4 pm NG price. The trading in DGAZ in the morning is based on the increase or decrease in the price of NG from the 4PM trade. The increase or decrease in NG as quoted is based on the earlier SET PRICE. Thus in times of "big" changes between the SET PRICE DGAZ trade and the 4pm DGAZ trade, DGAZ could actually go down even thought the quoted change in NG is down.
You bring out the issues that I was concerned about. However, if NG does have a relevant drop ( +.75 or more) DGAZ is still worthwhile over several months