Your "income" is  an estimated dollar value of the Steem tokens you will get based on the 3 day average of the Steem prices in the market. As the price fluctuates, so does your estimated earnings.

Keep in mind that until you redeemed your rewards, the money isn't really yours.  good to have that mindset in the unlikely event that someone downvotes your stuff.

<a href="https://musing.io/q/rahulwasan/pkywva455">View this answer on Musing.io</a><br />

The steem price is used to determine the worth of 

an upvotes and it is the upvotes that you get that determines the income you earned from a posts 

so when the steem price is falling then it will be normal for the income of the posts to decrease 

and if the steem price is rising it will also be 
normal for your income on a post to increase so 

that is why we all need to come together as force 
to make the market continue be bullish 

because when the market becomes bullish it will 
favour us as a steemian too

It doesnt always. It is based on the previous 3 days average of the price of Steem.
Therefore, if the price rises during the 7 day period, the value of your posts will increase, however, ifnthe price of Steem falls, so will the value of your posts and comments.
Sadly, the price of Steem has been falling recently!
I hope this answers your question. If you search the Steemit site, there are a number of posts explaining how payments are calculated.

Income form Steemit platform is based on the estimated dollar value of the Steem. If the value of steem goes down the estimated income will also comes down and if the value of steem goes up the estimated income will also be increase. 
During the recent period the Steem price is continuously coming down and therefore, income form steemit platform is also coming down.
We can only hope for the prices to go up in term of earning some handsome amount.