The Safari Jungle is a unique area in the game that contains different Pokemon in each of its three stages on different days - you won't find the same Pokemon on Monday that you did on Sunday, for example, but the Pokemon on Monday in the same stage are always the same. (Pokemon Battle Trozei uses the Nintendo 3DS calendar to determine the day, by the way.) You will unlock a new stage in this Zone by completing Zone 3, another by finishing Zone 6, and the final stage by completing the game, with the difficulty in each stage increasing also.