I've been struggling here as well. I've been approaching it with this in mind: Internship is your last year of generalized training so cover letters outline a wide range of experiences and highlight a few areas you could still use training in order to become a well-rounded clinician. On the other hand, postdoc is all about specialization. So instead of highlighting all of your awesome experiences, you want to highlight the experiences relevant to the area of specialization you're looking at....and then take it one step further and explicitly state how this particular site can fill in the gaps in your knowledge and training for that area. While also highlighting what skills you do have that will add to their program.
....I think?