When two threads talk about the same subject, yet doesn't get the same number of upvotes

Different boards, different titles, different number of views, and different ages.

Too many variables.

And even if there were two exact threads, the first would probably be more popular because most people would post in one thread, not always both.

Same topic, one just has a more interesting title.

I think it has to do with what board they're in.

Memes probably gets a lot more traffic than Story/art, honestly. Some people probably only browse specific boards, so they don't see both threads.