Price fluctuations do come into play, but another thing to remember is that there is a fixed rewards pool. Lets say hypothetically your post does really well and gets up to $345, and then doesn't have much voting action for the next 5 days until payout. As other posts that are also pending payout receive votes, the effect is that your post's relative share of the rewards pool shrinks.

My advice to newcomers- don't count your chickens before they hatch! You don't really have the reward until the payout has completed 7 days after you initially post an article, and many factors are at play which could cause the value to fluctuate until the payout occurs.

Just be grateful we have this amazing opportunity to generate and reward each other with wealth for our contributions. :)