May be the water contents of this veg having oxygen keeps it fresh and green

Well, iceberg lettuce is a hybride of lettuce and cabbage. Cabbage is VERY long-lasting, it can last all winter.

Next question: What makes cabbage last so long?

My answer is simple. Unlike romain lettuce(which is somewhat loose and open and therefore more expose to surrounding air) iceberg lettuce is wrapped tightly with each leaf hugging closely to the next.

From experience, when iceberg lettuce begins to brown on the outside, it's as simple as removing a few leaves to get to some fresh ones. Cabbage works in the same manner.

.... goose

Wikipedia:

Crisphead, also called Iceberg, forms tight, dense heads that resemble cabbage. They are generally the mildest of the lettuces, valued more for their crunchy texture than for flavour. Cultivars of iceberg lettuce are the most familiar lettuces in the USA. Bruce Church founder of Fresh Express, was responsible for popularizing the idea of shipping lettuce across the US continent from Salinas, California to the spots on the East coast. Using ice they carefully covered the heads of lettuce and shipped them year around and all the way as far as Maine, as the train pulled into each stop, folks would call out excitedly, "The icebergs are coming, the icebergs are coming!" The name would stick. Before that people had to depend on what you could grow locally and preserve from the gardens.

Those "Icebergs" were bred to be shipped from

California to the east!