Kwatts

You could say I am sitting in a bench provided that the bench has some parts to rest your body. Today such benches are common in parks. So you could say I am sitting in a bench.

i wouldn't say that I am sitting on a chair.

Now I am writing here. I am sitting in a chair and writing. I have an armed-chair. I find it is very difficult to say that I am sitting on a chair. I am sure there are people to say that I am sitting on a chair.