Pretty sure that you are just seeing other people’s votes mingling with yours. The post value needs to refresh, so particularly if you’ve been reading the article, all the time you’ve been reading new votes have been occurring. When you click to place your vote, it refreshes the counter in your browser and can create the seeming jump if a lot of activity was happening.
*to clarify- the mingling votes aren’t making them more powerful either, votes are determined entirely by Steem Power and vote strength percentage. If there was a big jump, one or more of the voters just had significant power.