Why is my staking reward dwindling?

I’ve had my sol staked for the last 57 epochs and upon review of the rewards history, I see that while by overall balance has increased by about 2.6%, the reward expressed as a percentage of the balance has decreased by 16.4%. I understand that there can be market variation but the change seems to exhibit a persistent downward trend so I suspect something more than market variation is going on. Are there reasons, other than market variation, this would be expected to happen?