This is a very good question and I'd like to help you with that.
There are more than one platform to use Steem on, Steemit is the most popular one though. On Steemit you will get a "slider" at 500 SP, which gives you opportunity to give smaller votes than the "regular" vote, which is so called 100% vote.
You can give a vote from 1-100%. Simply put, 50% vote will be 50% of the value, BUT you will lose only 50% of the VP you normally would (meaning a 1% VP loss instead of 2%).
Some users are giving a post small upvote, upvoting themselves bigger and maybe hoping they will attract more upvotes for their comment.
You can check the upvote size (and your VP) from services like steemd.
You can find your own from here https://steemd.com/@michalecki
Please remember to be extremely careful when clicking links left by other users!
Thank you, @suomibotti!!!
So basically it is a feature, I didn't know about it. Actually, it makes perfect sense.
I should make more research before posting question next time! ;-)