it does indeed sound strange that an obstructed view cabin is in a higher category than a non-obstructed view one.
However, as per NCL's web site the category H cabins have portholes, while cat. G cabins have windows. Virtually all cruise lines list porthole cabins as a lower category than window cabins, even if the latter ones have an obstructed view. "Norwegian Star" only has four cat. H cabins, as per the web site. This is also not unusual, as modern cruise ships have very few porthole cabins, which are usually located near the bow, which IMHO isn't a particularly great location, anyway.