You were actually right! In a way...originally those "backpacks" were all jump packs. Even devastator squads could have jump packs and every marine moved with individual deviation when they jumped. I guess it was so engrained in the artwork they couldn't just take them off and called them "backpacks" instead. If you have any questions about out-dated and useless knowledge for original version 40k, I'd be happy to rant about it.