A borrower who checks their credit at more than one place frequently finds three different numbers. The differences are not errors in most cases, because the bureaus maintain entirely separate files.
Reporting to bureaus is voluntary
No US law requires a lender to report account information to any credit bureau, let alone to all three. Furnishing data is a business decision, and it costs the lender something to do.
Some lenders report to all three, some to two, and some to none at all. Smaller creditors, local institutions and certain specialty lenders are the most likely to report selectively.
The result is that a file at one bureau can contain an account that simply does not exist in the file at another, which changes everything calculated from it.
Timing shifts what each file contains
Lenders report on their own cycles, typically once a month, and they do not all report on the same day. A balance recorded at one bureau may be weeks older than the one recorded at another.
Since balances relative to limits are a significant input to scoring, a snapshot taken before a payment posts looks materially different from one taken after.
Two bureaus can therefore hold accurate data that disagrees, simply because they were photographed at different moments in the same billing cycle.
Scores are models, and models differ
A credit score is not stored in a file. It is computed on demand by running a scoring model over whatever data the file contains at that instant.
Several model families exist, each with multiple versions, and lenders choose which to use depending on the product and how recently they updated their systems.
Running two different models over identical data produces two different numbers, so part of the spread a borrower sees comes from the model rather than from the file.
Consumer-facing scores are not always lender scores
Free scores provided by card issuers and monitoring services are real scores, but they are often educational versions or versions built for a different purpose than the one a lender will pull.
A mortgage application, an auto loan and a card application may each be evaluated with a different model generation tuned to that type of credit.
So a borrower can see one number for months and encounter a different one at the point of application without anything having changed in their behavior.
Disputes have to be filed file by file
Because the bureaus are separate companies with separate records, correcting an error at one does not correct it anywhere else.
Federal law gives consumers the right to dispute inaccurate information and requires investigation, but the process runs against each bureau independently.
Reviewing all three reports rather than one is therefore the only way to see the full picture, since an item missing from the report a borrower happens to check may still be affecting decisions elsewhere.