Currently the extradition of Assange has been refused on humanitarian grounds, as the US can't guarantee that they won't stick him in an ultramax facility and/or drive him to suicide. It's still facing appeal, but it's entirely possible the UK refuses to extradite.
I believe historically Sweden has been more willing to ignore such concerns when dealing with the US, which is why there would have been an incentive to fake charges that could get him moved to Swedish custody.