The later you vote the lower the proportion of the curation reward you get as a whole. It's a balancing act in terms of not voting to early or too late if you want the optimal reward.
All else being equal you want to be one of the earlier voters on any post to get a higher weighting on the curation reward.
Steem rewards you for curating content through votes. The earlier you vote the better. If you vote too late then steem punishes you for it marginally. To prevent bots from early voting on everything steem penalises votes in the first 30 minutes.