I'm still curious (though I don't know if we'll ever find out) why not everyone who had credit report data at Equifax was affected. It's not like 25% of people which a credit report have no data at Equifax, because most active account data (though not application data) is reported to all 3 bureaus.

Maybe it was as simple as that it took many weeks for the hackers to retrieve all that data, in many smaller downloads (whether they couldn't download 143 million record that fast, or simply tried to download slower to hide what they were doing), and the hackers got cut off before they got absolutely everyone's records. (Or maybe the hackers were never expecting that much data, and ran out of storage or out of time before they got all the data. ) And Equifax may have an idea at which point they got cut off, but Equifax is not explaining what sequence the data was being stored in or stolen in. It could be the hackers got the accounts most recently updated, most long-ago updated, most recently created, most long-ago created, who knows. They could have been in just about any sequence you could imagine.