As has been pointed out several times your placing on the leaderboard depends on a combination of things. However.. I tend to agree with twalk in so much as looting ships seems to be a major consideration.
I have consistently been mid-table on the leaderboard. Until 3 days ago when I made the decision that I needed to upgrade and just stuck to the islands. Since then I have dropped off the league table altogether.
So as far as I can see it seems to be the deciding factor.
btw - Have now upgraded and am broke