I think there are multiple ways a vote's value can change in the 7 day period between creation and payout. Some are:

• when other votes are added to (or removed from) the same content
• when the size of the reward pool changes (large investors either come to or leave from the platform)
• when the way the reward pool is being used changes (if other content gets an influx of votes)

The FAQ may offer some clues in the section "Why do the earnings for my post go up or down?":

"The amount that is shown next to a post is a Potential Payout in BLURT. This is an estimated value of how much BLURT the post will make based on the votes that have occurred so far and the economic factors such as the reward pool and recent claims. Note that these are not USD estimates and hence should not fluctuate very much unless some voting activity has taken place. Depending on various factors, this value can go up or down until the payout window closes:

If a post receives more upvotes, the potential payout of the post can go up.
If other posts receive more upvotes, the potential payout of the post can go down very slightly.
If upvotes are removed from a post, the potential payout of the post can go down.

It is important to understand that global economic changes, such as voting activity increasing or decreasing across the Blurt chain, will have a small effect during the 7-day voting period on an individual post; the largest effect is from the votes on the post itself."