Other or not chosen can be harder to find as they are disguised in various ways, like a fake pine cone, a fake bolt, hidden in a log, or some other container not the usual used for the other sizes. They often pass as some common or natural object that most would discount. Also, these are not as common as the other containers. The two categories (other and not chosen) are combined into the 500 for your qualification. The actual D/T rating is more like 2/2 but the posted D/T rating reflects the difficulty in finding enough other and not chosen and the amount of terrain one has to cover over time to accrue enough finds to qualify.