The reason why is because the voters with the most steempower receive it because they have promised to lock up x amount of steem for 2 years, making it so they cannot sell large portions of their locked away stake. Due to this added risk, one would assume this entices people to lock away large amounts of their stake in return for voting stake (which essentially enables you to throw magic crypto gold at users who you think produce quality content).

So if someone can make a post go up by nearly $100 from their upvote, it is because they have locked away a lot of steem (probably over $40k worth --guessing). So that person is someone you would probably want to enjoy your posts :D