Sometimes they wiil see it & it will only go into their subconscious & then they will think of it later (30 seconds is enough to have a brain fart) believing that they came up with it themself, other times they do it because they can & they don't care, or they may not like you for some reason & are trying to piss you off, but remember all of their upvotes belong to you! lol.
Don't let it get to you, I see it all the time on here, most days, both with mine & other people's.. that's life.
Just enjoy your time here.
If you only get one upvote just remember like somebody else said.. you made someone else laugh, it might of made their day for all you know so it was worth it 😃
Sometimes though people will just think of the same thing as you, I had to scrap one meme before I uploaded it because of that my fault for leaving it to long before creating it.