Your voting power decrease with every vote that you cast. Which means your second vote is not as powerful as the first one and so on.

Your voting power is reset everyday by 20%. Which means if you have used 100% of voting power, it would take 5 days to recover it. You can check your voting power status at www.steemstats.com by putting your username in the field.

For more visit https://steemit.com/faq.html#What_is_my_voting_power