People keep throwing this number around like it means something. Whether 15 exclusives or 5 or 50, it means nothing until we see those games.
Case in point: the Wii U, some six months after launch, already has at least 8 exclusive games (NSMBU, Nintendo Land, ZombiU, Lego City Undercover, Sing Party, Rabbids Land, Game Party Champions, Family Party: 30 Great Games) and at least 2 home console exclusives (Monster Hunter 3 Ultimate, Tank Tank Tank). Several of these are "original IP" as well. Now consider how many of them are just bad games. Hint: it's half of them. Not only is there no guarantee that some of Microsoft's 15 exclusive games won't be bad, it's pretty much a no-brainer that they're going to count every possible game in their number of "exclusives" no matter how bad it is. You can bet the executives aren't saying, "Those five games will be very bad, only tell people there will be 15 exclusives so they don't get their hopes up."
Now consider that the Wii U's 8-10 exclusives is not including "special edition" re-releases of older games or downloadable exclusives, and there is again no guarantee that Microsoft is not including such titles in their "exclusives" count.
I'm just trying to make the case that saying your console will have 15 exclusive games in its first year means nothing, and the Wii U is proof, because for all its technical exclusives, it has had the most embarassingly dry release schedule I've ever seen for the past 5 months.
Unless by their first year, Microsoft means the ~6 weeks between the XB1 release and the end of 2013. That might be impressive. But that's not how I interpret the statement.