100^2 + 100^2 != 200^2

(100 + 100)^2.

So lets say your first 100 voting power guy upvotes a post (lets assume hes the first one). The post will now have 100 voting power voting for it and get 100^2 in rewards.

Now lets say the second guy comes along and also votes for the post. Now, the post will have 200 voting power voting for it, and get 200^2 rewards.

So if the first 100 voting power vote gives him $1 in rewards, the second will give him an additional $3 (for a total of $4) and the third, if there is one will give him an additional $5 (for a total of $9)

But if these three guys all voted on seperate posts alone, they would only be assigning $3