Mixed credit reports are caused when the credit bureau places information belonging to another consumer on your credit report. If you’ve got a common name, there’s a chance that your credit report could be crossed with another consumer with the same name. Since the credit scoring software is not able to distinguish between your credit data and credit data belonging to another person, it will "score" the information even if it’s incorrect. Consequently, if the other consumer with the same name has poor credit, this information will now appear on your credit file resulting in a possible denial of credit.
The reason mixed files are so hard to correct is because the lender typically is not sending in incorrect information. The problem is being caused by the credit bureau inadvertently co-mingling data belonging to two consumers and placing it on one credit report. So, the lender cannot fix the problem because they’re not causing it. Additionally, the credit reporting agencies are under no obligation to proactively investigate the information on your credit reports to determine if it’s yours or if it belongs to another person with the same name. You have to pull your own credit reports, review the information and then file a formal dispute with the credit bureaus if you find data that you believe is incorrect or belongs to someone else.
According to myfico.com, additional reasons for incorrect data in credit files include:
The person applied for credit under different versions of their name (Robert Jones, Bob Jones, etc.).
Someone made a clerical error in reading or entering name or address information from a hand-written application.
The person gave an inaccurate Social Security number, or the number was misread by the lender.
Sr.'s and Jr.'s living within the same household get account information crossed
Loan or credit card payments were inadvertently applied to the wrong account.