Jungle originally contained 48 cards in Japan; however, the English version was expanded to 64 cards since Wizards of the Coast decided to make both holographic and non-holographic versions of the cards that were originally only holographic. This was a recurrent event that Wizards used in many sets to make the sets larger, as well as to prevent packs from guaranteeing a holofoil card.
Jungle was originally planned to be released in North America around August (and rumored as late as October). However, since Topps was releasing their Pokémon trading cards early in the summer, WotC decided to push Jungle far ahead of schedule to June 16. Within two weeks, 1st Edition Jungle was sold out. Unlimited Jungle was released shortly thereafter.
The 1st Edition print run of Jungle also accidentally contained the extremely rare 1st Edition Promo Pikachu. Word is that there was 1 of these to every 10 boxes, making it very valuable.