In the last steem update steem introduced this 12hr thing. Most people were seen upvoting content when close to payout time, so that they can get an instant payout. To solve this unfair voting techniques steem made a change.
We can vote and get paid in the last 12hr, but the voting stength reduce inversely with time.
12 hrs left - 100% curation reward
6hr left - 50% curation reward
0hrs left - 0% reward