Yahoo Inc. and the National Football League didn't hesitate to trumpet the numbers from the partners' first-ever free live stream of an NFL game on Sunday. Touting the stream as a "truly historic event," they revealed that the early-morning faceoff between the Buffalo Bills and the Jacksonville Jaguars at London's Wembley Stadium picked up 15.2 million unique visitors, a third of whom came from outside the United States. 

In the lead-up to the event, Yahoo had reportedly charged around $50,000 for an ad spot and guaranteed advertisers around 3.5 million U.S. streams. "How could Yahoo deliver an audience for that?" Re/code's Peter Kafka asked after the fact. "Here’s one way: By streaming the game to everyone who visits Yahoo’s home page, whether they want to see the game or not."

Counted among the "viewers," then, was anyone who checked his/her email at Yahoo via the front page, where the game played automatically without sound (as Kafka pointed out, Yahoo says an average of 43 million people visit that page each day). It's unclear how much this could have inflated the game's audience to 15.2 million.

But even taking the figure at face value, laymen would be mistaken to regard it as the equivalent of a Nielsen TV rating. 

Standard TV ratings are calculated differently than digital ratings, and according to Nielsen, the digital audience is always going to seem inflated in a side-by-side comparison. 

"TV measurement is an average commercial minute, meaning we add up all of the minutes of viewing throughout the show, and then we average the audience over each minute," Megan Clarken, Nielsen’s executive vice president for global product leadership, told International Business Times in an interview Thursday. "And digital ratings, today in the current guise, is an impression count. And so they’re not the same thing."

In other words, there's no way to know whether the 15.2 users touted by the NFL and Yahoo truly watched the stream (and the ads that came with it) or merely glanced at the feed. 

"It is not possible in the current environment just taking digital impressions and adding them to your average commercial minute. Your digital impressions will be way, way, way bigger than what your average commercial-minute audience was," Clarken added.

CNN’s Brian Stelter crunched the numbers of the NFL live stream based on total minutes, and reported that, by those metrics, the game averaged 2.36 million viewers, a figure confirmed by an NFL spokesman.

Nielsen, meanwhile, is preparing to launch a tool that would count TV and digital video with more consistent methodology; it has said that the industry adopt comprehensive and comparable measurement across all platforms, rather than "confusing metrics."

But until that happens, the NFL and Yahoo can wave Sunday's streaming figures around, comparing them to TV viewership.

“We’re thrilled with the results of our initial step distributing an NFL game to a worldwide audience and with the work of our partner, Yahoo,” Hans Schroeder, the NFL's senior vice president of media strategy, said in the press release. “We are incredibly excited by the fact that we took a game that would have been viewed by a relatively limited television audience in the United States and by distributing it digitally were able to attract a global audience of over 15 million viewers.”