Why the World Cup numbers differ in ESPN press releases from the reported Nielsen numbers

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tl;dr: press releases are for bragging rights, Nielsen ratings are for facilitating the buying and selling of advertising.

Last Monday (June 23), ESPN sent out a press release saying that  World Cup match between USA and Portugal the day before averaged 18.2 million viewers.

Then the Nielsen numbers came out showing the telecast averaged 13.8 million viewers, nearly a 4.5 million viewer discrepancy. At TV by the Numbers we heard cries that we were reporting the wrong numbers. We’re not, but why are the numbers different?

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