A resume is fact-based and brief. A resume is a list of your professional skills and experience and a cover letter, on the other hand, is more detailed and informative. It tells why you are the best person for the job. Candidates make this mistake because they don't know the difference and make their resume and cover letter and mix the information. That's why now mostly people hire professional resume and cover letter writers so they can get the professional resume and a cover letter written in the correct format.