I think this is easier to explain from the beginning. Let's say I'm an employer and I pay you, a W-2 employee. I pay my employer part of your SS and withhold your employee part and remit both to the IRS. When you file your taxes, the IRS matches your return SSN to the withholding SSN to ensure that the appropriate amount of SS was withheld and paid by the employer. If there was any kind of "typo", then they will show that you didn't pay any SS tax. They'll send you a bill for at least the withholding part, and if it looks like you're self-employed than for the employer part as well. If this was a mistake because of a typo, then you'll alert them that indeed you did have your SS tax withheld and eventually they'll be matched and that money the employer remitted will be connected to your tax return.
If you're using a fake SSN or a dead person's SSN and your employer assumes it's good they're still going to withhold your SS tax and pay the employer part and remit both to the IRS. That money will then sit at the IRS waiting for someone to file a return with a matching SSN. If this never happens, then someone was obviously using a fake/invalid SSN with their employer. The vast majority of the time this is someone working without an appropriate work permit. The money goes into the SS fund just like every other contribution, and the undocumented worker just helped out the SS trust fund without getting anything out of it. It's trivial for the IRS to count the money in this status and report it, which they've done here. It's not there because of typos!
