It works as expected. There are millions of SP out there and you have only small share of it. So at the time of voting, you are using your share to decide about potential payout. Your impact is proportional to your SP (compared to all other voters in a given time period). Even if your vote doesn't affect potential payout in a visible way (i.e. it's less than $0.005) it's still something.
As they say: "Many a little makes a mickle".
By the way, voting for post older than 7 days doesn't affect payout for that post nor your voting power.