What makes an agency "nationwide" is a statutory test rather than a reputation. 15 USC 1681a(p) defines a consumer reporting agency that compiles and maintains files on consumers on a nationwide basis as one that regularly assembles, evaluates, and maintains, for the purpose of furnishing reports on consumers residing nationwide, both public record information and credit account information from persons who furnish that information regularly and in the ordinary course of business. Both limbs are required. That category is what the Act's heaviest consumer protections attach to, and Congress described it rather than listing the companies that satisfy it.
A second category is invisible to almost everyone and covers files that have nothing to do with lending. 15 USC 1681a(x) defines a nationwide specialty consumer reporting agency as one maintaining nationwide files relating to any of five things: medical records or payments, residential or tenant history, check writing history, employment history, or insurance claims. Those files are consumer reports and they carry the same disclosure and dispute obligations as a credit file. Anyone who has been declined for an apartment or quoted an unexpected insurance premium has probably been the subject of one.
A third category explains the report a mortgage lender actually buys. 15 USC 1681a(u) defines a reseller as a consumer reporting agency that assembles and merges information from the database of another agency or of several agencies in order to furnish it to a third party, and that "does not maintain a database of the assembled or merged information from which new consumer reports are produced." A reseller is therefore a middleman with no file of its own, which is what a merged three-agency mortgage report is. It also means a reseller cannot fix anything at source: the underlying data belongs to the agencies it drew from.
The data flow runs from furnishers, and furnishing is voluntary. The companies that send account data to the agencies are called furnishers, and they are typically lenders, card issuers, and collection agencies. Nothing in the Act requires any of them to furnish anything. What 15 USC 1681s-2(a)(1)(A) does is prohibit furnishing information the furnisher knows or has reasonable cause to believe is inaccurate, and the Act then layers several specific duties on those who do report: under (a)(3), once a consumer disputes the completeness or accuracy of an item directly with the furnisher, the furnisher may not furnish it to any agency without a notice that it is disputed; under (a)(4), a furnisher that regularly reports must notify the agency when the consumer voluntarily closes an account; and under (a)(5), a furnisher reporting a delinquent account placed for collection or charged off must, within 90 days, notify the agency of the date of delinquency.
One furnisher duty explains something readers find genuinely puzzling. 15 USC 1681s-2(b) sets out what a furnisher must do after an agency relays a dispute: investigate, review the information the agency provided, report the results back to that agency, and, where the investigation finds the information incomplete or inaccurate, report those results to all other nationwide agencies to which it furnished the information. That last limb is why a correction obtained through one agency tends to appear at the others, even though the consumer only disputed once. It is a duty on the furnisher rather than a courtesy from the agencies, and it does not reach agencies the furnisher never reported to.
Why your three files differ, stated as the institutional fact it is. Because furnishing is voluntary and is decided per agency, a creditor may report to one, two, or all three, and may change that. Timing differs, since each furnisher reports on its own cycle and each agency processes on its own. Public record collection differs. And the statutory test at 1681a(p) requires each nationwide agency to maintain public-record and credit-account information on consumers nationwide; it does not require them to maintain the same information. So three roughly similar and rarely identical files is the expected outcome of the system's design rather than a sign that something has gone wrong. The consequence is that "my credit file" is a set rather than a document, and a lender's decision turns on whichever member of the set it happened to buy.
What this page deliberately does not cover. The contents of the file, how long items may stay in it, and who is allowed to see it belong to the report itself. Scores are produced by modeling companies rather than by the agencies, and the agencies run those models against their own files under license. The procedures for disputing an item and for freezing access are separate statutory machinery with their own deadlines.