The reason for the difference is that the 2nd biggest holder is likely poloniex, and they don't stake their coins at all.
I never understood why they wouldn't stake and take a 10% cut of coins earned or something.
They make more money and they help decentralize the coin that they are risking their asses on by accepting deposits...
Somehow they're already making enough money that the extra 10% here just fell below their radar? I dunno. It is surprising that they wouldn't stake on reasonably valuable POS altcoins that they support.