Yeah, I noticed that as well. I don't know why will anyone be upvoting themselves any ways, when they can vote for others and still get their curration rewards.
I like to see people judge my work by voting or not voting for my post, that way I can know how to improve.
There is no other reason but it's because of greediness that some would curate themself.
Some curate themselves to boost thier post but too much of self Curation is bad
I am also a victim of self Curation and I do do that to encourage my content and being proud of myself even though nobody curate me