The original version of the slogan was "If it's in the game, it's in the game", meaning that EA's sports-based video games captured everything interesting about the sports they represented.
Eventually the slogan became so familiar to gamers interested in games of this sort that EA truncated it to just the consequence clause.
muru asks for a source.
Here's the website of Jeff Odiorne, who with with Michael Wilde developed the original campaign at Goldberg Moser O'Neill and Riney. A brief video about it is at the bottom of the page.
Here's a story from Ad Age about Odiorne and Wilde's work after they left Goldberg and started Odiorne Wilde Narraway Groome with EA as their principal client. It includes this:
Goldberg won the account in August, just in time for Odiorne and Wilde to create a campaign to launch the EA Sports line for the Christmas season. The two came up with the position of authenticity that was based on not only a knowledge of sports but a passion as well. Their tag, "If it's in the game, it's in the game," was deceptively obvious, [Doug ]Transeth [VP-sports marketing at EA] adds. "It was so simple," he says, "it scared us." [my emphasis]
And here's a record of the trademark history, drawn from the US Patent and Trademark Office.
And just for lagniappe, to demonstrate what Odiorne and Wilde were after, here's an example of a fan praising the attention to detail in a recent edition of Madden.