Same thing happened to me when I was using steemvoter or a auto voting tools from buildteam.I can't remember the exact name of the service I was using.But things like that are supposed to happen when you use some tool for voting or use auto voting system to vote content for you.
The best is not to use that kind of service if it bothers you.It is not vote stealing exactly as you have given them the permission to upvote on behalf of you.
Dear @philipkavan
Thank you for your comment.
I just learned that information about those upvotes are displayed in FAQ section. I'm not sure if that means that I have given anyone permission to upvote on my behalf. The truth is that it's very tricky way of hiding those expenses.
I strongly believe that each customer should have "black and white" all informations about upcoming costs being displayed during registration. But then, I may be wrong.
Thanks again for taking the time to share your thoughts with me.
Yours
Piotr