I think some people feel they have made a lot of friends in both the "dog side" and the "cat side" so they post in general so that dog or cat people who don't normally go to the other side may see it. Sometimes people have posted that they are posting something in the 'wrong' section, i.e. a dog health thread in dog general, because they feel they may get more views/input. Some people have posted a memorial for a pet in pet general because they feel not many will go to pet memorial. Sometimes they post pictures of all their pets, but don't post in in pet general because they feel not many will see it there. Sometimes it's probably an accident. Probably there are many different reasons for it

As for me, I'm more likely to miss a thread in general than anywhere else, just because it scrolls by so fast.