the gemara answers that when david says he got up 2 ashmurot, it means before other kings that get up in the third hour, and so you get 8 hours which is the same as two ashmurot, but if you think about it its not the same as 8 hours, because with the cheshbon of shaot zmaniyot, night hours are somertimes longer and sometimes shorter than day hours, so if we add two daytime hours to six nightime hours some parts of the year it wont be two ashmurot worth of time, because the ashmurot are at night, so if we go with 8 nightime hours, it would be more than six plus two?
Marcus Abisror, Long Branch
This is an intriguing question! But isn't it so that if you consider Sha'os Zmaniyos, there will always be 6 hours from Chatzos until dawn, that is to say 6 Sha'os Zmaniyos? And, actually, the same is true about the daytime hours, for even in the winter when two Sha'os Zmaniyos are, let's say, 90 minutes, this would still be considered two hours since we are halfway or more into the second hour.
Alternatively, we could say that since in the great majority of the year (spring, summer, fall) David ha'Melech arose two full eight-hour periods (or more) before the other kings, his description of his sleeping pattern was justified. Therefore, even though in some months we might not have been able to derive that habit due to the short hours, nonetheless we can describe it as such since we are addressing a yearlong pattern of behavior.
Shimon Brodie