Why rewards are different each day?

The rewards can vary each day, they are not fixed. Every day only 50% of the nodes are generating rewards. This is how Elrond blockchain works. So there are days with higher rewards and days with smaller rewards. But on a timeframe of 3-4 weeks you will see the reward rate will be closed to 18%-19%.

Here is the longer explanation:

Each shard has 800 validators assigned to it. 400 are eligible and 400 are waiting. The ones waiting are showing with a clock in the explorer.

Round time, is the time measured in seconds required to create successive blocks on the Elrond blockchain (6 seconds). In each round, a new consensus group is selected using a randomness source that can be neither predicted, nor influenced. The consensus group consists of 63 members. So, it follows that not all validators are in a consensus group at all times. Validators who are in consensus must do the work required to prepare and sign blocks, and must complete the work within 6 seconds. A rating system is used where the rating is increased slightly each time work is successfully submitted, or the rating is decreased slightly if the work is not submitted on time. The default rating for new nodes deployed to the network is 50. Rating will increase over a few days until it will finally reach 100. If a validator node goes offline, it will obviously not be doing it’s work, and so the rating will decrease. If the rating is ever decreased to 10 or less, the validator is taken out of service and a new one is taken from the validator waiting queue.

The epoch time is 24 hours. At the end of each epoch, 80 validators from each shard (total 320) are randomly selected and shuffled randomly into each of the 4 shards. The validators that were shuffled are put on the shard waiting list (showing with a clock in the explorer). Validators will spend 5 days on the waiting list until being selected again to be eligible. During the waiting time, the validator details page in the explorer will show “not in consensus”.

Over time, the amount of time spent in the shard waiting list and the amount of time spent in the shard working will average out to 50/50.

This is the normal operation and lifecycle of a validator node. As long as the rating is increasing toward 100, the node is doing its job and rewards are being generated and credited at the end of each epoch where the node was active and not waiting.