I have trained a lot of pilots over the years and would have to say it is hard to get the two confused. I often test my students descent management skills by getting them to clearly set a number of 'gates' that they are trying to achieve all the way through to landing. I can then monitor if they have any idea about profile planning and in the physical control of the actual descent whether they can manage that plan. To me being behind the aircraft is a student who just doesn't anticipate what needs to be done next and takes to long to action it, so it is possible to have poor descent management due to being behind the aircraft or becoming behind the aircraft because of poor descent management.