I wonder also. My husband and I have the same accounts and the history is so different. I have asked this question to Equifax before but have not got any response that I understand. For example our truck loan with Wells Fargo is showing 3 30-day late back in 2003/2004 on my credit but showing 17 30-day on my husband. Same account. I brought this to the attention of Equifax who verified with Wells Fargo during this time I was 3 times late and my husband was 17 times late. It makes me question how they verified it. We have never been 17 times late on the vehicle, so I know this is an error the report but I have disputed this several times with Equifax and they always come back verified. I have called Wells Fargo and apparently this is how they work it. The note is due on the 25th - sometimes I paid it 10 days late but since it was due on the 25th it you pay it on the 1st of the next month they count that as 30 days late - really in my head on 6 days late. Ok so this is how they report - then why was I only 3 times 30-days late and my husband 17 times 30-day late.
I felt like I was beating a dead horse and since it occurred in 2003/2004, I decided to just not worry with them anymore. Besides when we apply for credit together at the bank or credit union, we point out the differences on our credit report (being that it is the same account) and our bank takes that into consideration and knows that something is not right.
I am sure it hurts my husband's score some but he is in the upper 600's. Also, I am afraid that if I pursue this they may change my report from 3 3-days late to 17 30-days late, so I am going to let sleeping dogs lie on this account.