Im not too sure but i have some theories...

Theory 1: Payouts occur every 24 hours, if someone votes on your post while they have 99% voting power... and then they go on to vote on 999 other posts during the same day. Then their voting power is reduced since the payout is split into many other posts. I believe this is to prevent spamming of upvotes and promote upvoting on only good content. You may have 99999 steem power, but if you have 0.01% voting power then well not all of your steem power will be effective when upvoting.

Theory 2: idk i cant think of another one...

Random fact you can check your voting power on https://steemd.com/
Dont know how to find your account?
Use this quick tutorial:https://steemit.com/tutorial/@suchnewb/simple-tutorial-how-to-check-your-account-s-stats-on-steemd-com