@andrewharland is correct, the % is holding steady because your growth is in line with the overall growth in the pool. But the SP backing that % is growing. 1% upvote from 25k SP is the same as 10% upvote from 2500 SP.

Another way you could look at it is that if your shares weren't increasing on pace with the pool, you might eventually drop below 0.50% and move into pool 2 where you would get a higher % upvote but from lower SP, so actual weekly value would be close to the same. (About 5% lower on average.)