Piggy-backing on Barrie's answer, you could you say that it depends on how relaxed your posture becomes. You sit **in** furniture where you lounge, and sit **on** seats where you sit more upright.

So, I'd be more likely to sit ***on*** a bench or ottoman, and more likely to recline ***in*** a rocker:

![enter image description here][1] ![enter image description here][2]

![enter image description here][3] ![enter image description here][4]

It might also depend on the verb you use, too.  Consider that blue divan: depending on how much I wanted to relax, I might either sit ***on*** it, recline ***on*** it, or lounge ***in*** it...  




  [1]: https://i.sstatic.net/Pnq8D.png
  [2]: https://i.sstatic.net/giWcn.png
  [3]: https://i.sstatic.net/yvg3I.png
  [4]: https://i.sstatic.net/CVxb0.png