I'm new, but I was guess it's because blindly swapping upvotes if not curating the site, which is the purpose of upvoting and commenting. An unvote, I believe, signifies that you find value in a post. Whales are the people with lots of money in steemit and therefore lots of voting power--the founders, for instance. All upvotes are not created equally. Just do a search for whales on the site and you'll learn about @ned, @dan, @blocktrades and a few other powerful swimmers. I hope they swim my way.