Is the rear wiper a factory part? An aftermarket part MIGHT be too long. Those reference guides are not always correct. 
Also, in my experience, aftermarket arms can be prone to rapid rusting and binding. In that case you would be correct, it may not be laying 'flat'.
On some other makes, the joints can rust. In that case, working some lubrication into the wiper arm hinge will help. Use a penetrant first to free it up, flexing the hinge to help it get into all the needed areas. Then follow up with a drop of regular oil.
Also, in my experience, aftermarket arms can be prone to rapid rusting and binding. In that case you would be correct, it may not be laying 'flat'.
On some other makes, the joints can rust. In that case, working some lubrication into the wiper arm hinge will help. Use a penetrant first to free it up, flexing the hinge to help it get into all the needed areas. Then follow up with a drop of regular oil.