It seem that Steem Dollars fluctuate based on curation and value. So if the post gets a lot of attention but by the end of the 7 days that you can cash out it receives less attention, then the curation value goes down and the value of Steem Dollars go down. Also, i Believe if the value of Steem as a coin goes down, it also decreases your value. And something I've just read is really interesting.
Theres is something also that is basically like an early commenter penalty. If you comment before the post is at least 30 minutes old, you will get a fee and it pays out less than you would have for waiting. If you comment 1 minute after a post is published you will only get 1% of what you would have gotten. 10 minutes will give you 10%, 20 minutes 20%, and so on and so forth. If the post is 30 minutes old and more, you'll get rewarded normally. Also, you only get Steem Dollars from your comments if people upvote them.
Hope this helped!