There are a few reasons that this is possible. You lose voting power when you upvote comments or posts. It takes time to re-generate your voting power. This is so people do not just auto Upvote every single post which was allowed early on. Now you can Up-vote at 100% about 10 Times per day and maintain your 100% power. If you upvote to fast your power will drain. As you get more Steem Power you will get a slider bar which will allow you to select your voting power per Upvote. You can then select 10% so effectively multiplying how many upvotes you can cast to 100 per day. The amount of rewards granted per your Upvote is based on your Steem Power Total And Your Power % of Upvote.