Update.
The La Crosse C85845 accuracy is fine!
The problem was that I had installed the outdoor sensor above the back door (north facing) in such a way that about one inch of it hung below the top door jamb (the door opens inwards, so this was not an issue).
Even though the back porch is unheated, there is enough heat escaping from the house through the porch and out the back door.
Because the sensor was actually hanging below the jamb, it was picking up a stream of heat coming through the door.
After moving the sensor to a higher position, the temperature on the La Crosse matches that on the RS unit, with its sensor having always been mounted higher on the door frame.
I guess this should be a hint to me that I need to install more weather stripping on the doors into and out of the porch, as well as a window that opens out onto the porch.
Note that this porch is fully enclosed, with one door from the house and one to the outside, plus one window from the house and one to the outside.