In the cover letter you write why you would like the job, looking to the future, seeing yourself in the position you are applying for.
In the resume, you are writing your history. Where did you go to school, where did you work before. What are you good at, what do you like to do in your spare time.
And, as Gandalf says, each story needs a little embellishment, so your resume might also be a better story than it actually is. You did graduate from your school, but you don't mention how you had to struggle to make the final exams. And in your previous position, maybe you only did that part of the job description once, but why mention it.
Sometimes that's not a real problem, since you do have to get used to your new work environment and can learn new roles. But don't embellish your story too much. Otherwise people will get dissappointed with your story and with it, also with you. And when other people don't believe your story, they will not listen to you and your stories anymore.