When you cast an upvote, you are delivering a certain amount of 'votes for reward' or 'rshares' to that post. During the period of HF19 and HF20, their conversion value to SP/STEEM/SBD at time of payout was relatively consistent relative to stake (what is referred to as linear rewards curve).

Now instead of the value relationship between vote points and reward being linear, it follows a quadratic curve that is being referred to as convergent linear rewards curve or CLRC. You need to stop thinking about your upvotes having a fixed value. Your are delivering vote points and their value is based on the total distribution of vote points throughout the blockchain.

The greater the total rshares delivered to a piece of content, the more favorable their conversion ratio into SP/STEEM/SBD at payout. The various dapps show an estimated payout value based on the current state of pending content, but it can change a lot more between vote and payout than previously.

In particular, each additional vote increases the rshares on something and improves its total payout favorability. In effect, each additional vote increases the pending value of all votes already on the content. This will not always be apparent when you vote, because many other votes may have gone out to other content elsewhere at the same time, and the ratios are based on relative totals.

This is also why many have been specifically targeting trending content with their downvote pools.

For more on the topic: https://busy.org/@steembasicincome/sbi-in-an-hf21-world-part-2-convergent-linear-and-downvote-pools