There is no mystery about this.

When you click to vote it forces the dollar value for the votes for that post to update. Before you clicked it you may have been looking at an older value.

As for why the dollar value of the vote tally varies from minute to minute it is not that complicated.

The value of each vote is dependent on how many people vote each day and the price of Steem.

This is because the amount of Steem produced each day is fixed and hence a fixed amount of this is available as payment for posts via voting.

The values are only estimates and are likely to vary over time based on the difference between the number of people actually voting and the prediction system which determines the value of the vote.

In addition to this the actual moment to moment value of Steem will also vary which will also affect the figure.

To be honest given all the variables it surprising that it does not change more often.