There are others with higher SP that voted around the same time as you did, so that likely explains why it doubled in value :)

When you have around 485 SP (Steem Power), you'll get a voting slider to adjust the strength of your vote. If you've below 485, that means you're currently voting at 100% voting strength everytime you do (which cuts about 4% of your total power). You can check your voting power at https://steemd.com/@yahialababidi . Think of voting power as something like mana points, and it replenishes by 20% everyday.

But don't fuss about it , just use the platform after some time and you'll get it :)

Edit: this is how a voting slider looks like.

As for auto-voting, there are services where u can set to auto-vote your favourite authors to support them whenever they publish a post.