I will explain. It used to be that super massive astronomical objects gave "super massive" pay outs, which included the objects at the centre of the galaxy. The credit reward for scanning these objects was obscenely high - I believe high enough that scanning these objects on their own was enough to make elite rank on their own at one point. This was adjusted very shortly after launch so that it is much harder to earn Elite rank in exploration. Zion earned the rank when this payout was still included in the game.
It seems that most of the higher ranks in both trading and exploration multiply the previous ranks credits by 3.3, and as I hit Pioneer rank somewhere around 30 million, I would expect Elite rank to be around 100 million credits.
