Bonjour Paul !

I posted the same question a while ago on the forum and here is Monsieur Gruff’s reply ->

“Prepositions have no logic to them. "Dans le dos" is "on my/his/her back" (the article doesn't match either!).

Also, don't assume that "sur" will replace "on". If you tell someone you're "sur le train" they'll think you're on top of a train, not inside it.”

Well, I too encountered a similar case- 

Je m’assieds dans le canapé. (which is grammatically correct)

Here, “dans” is used to indicate the coziness aspect associated with a sofa.

Non-native speaker so apologies if I get corrected by Madame Cécile.