My view is the Whales simply cannot post enough content to keep their percentage of the reward pool consistent. 75% of the reward pool goes to authors. With more people at the lower levels posting, collectively they are taking more of that pool. Hence their percentage of the whole goes up over time.
The same holds true for the dolphins. To start, some minnows hit dolphin level which adds to that total. Plus with almost 1,500 dolphins, most of whom are posting, again that takes a chunk of the reward pool.
It is a distributive system.