Why are my staking rewards decreasing?

Sorry if this has been explained elsewhere but I tried researching this and couldn't find an answer.

I started staking Ethereum back in April and have seen my weekly rewards steadily decreasing and I'm trying to understand why. It started April 13th where I staked 0.27848 ETH in Kraken and then started seeing the following rewards over the subsequent weeks:

* April 17: 0.00023 * April 24: 0.00038 * May 1: 0.00037 * May 8: 0.00036 * May 15: 0.00035 * May 22: 0.00034 * May 29: 0.00034 * June 5: 0.00033 * June 12: 0.00033 * June 19: 0.00032 

And so on in this way until this past weekend where I received 0.00027. Can anyone explain why my rewards are steadily dropping if the staked amount never changes?