When Microsoft announced Rise of the Tomb Raider as an Xbox-exclusive last summer, some fans scratched their heads and wondered: Why? After all, the 2013 Tomb Raider reboot was released across Xbox and PlayStation consoles, as well as PC. Now, developer Crystal Dynamics boss Darrell Gallagher has expanded on the nature of the deal, what it means for the game, and more.

Speaking with Game Informer, Gallagher says the relationship didn't happen overnight. In fact, it's "been a long partnership" that actually began, in a way, back in 2008 when Crystal Dynamics brought Tomb Raider Underworld to the Xbox 360 with some exclusive DLC.

This was followed up with Lara Croft and the Guardian of Light in 2013, which was released first on Xbox 360 through Microsoft's Summer of Arcade promotion. As a result of these previous working relationships, the Rise of the Tomb Raider Xbox exclusivity deal represents a "natural evolution on where we started," Gallagher said.

Microsoft will publish Rise of the Tomb Raider and handle marketing and distribution. Overall, Microsoft working on the project helps Crystal Dynamics "achieve what we want with Rise of the Tomb Raider; to deliver the best game that we can," Gallagher said.

The interviewer went on to ask Gallagher if he thought that Crystal Dynamics' partnership with Microsoft was so deep that Rise of the Tomb Raider could not exist without it. Gallagher said he doesn't think that's the case.

"I wouldn't say that," he said. "If you look at Tomb Raider, it's been around for 20 years, so I do think that Tomb Raider would still be around [without Microsoft]."

Gallagher explained that partnering up with Microsoft helps boost the profile of the Tomb Raider franchise to new heights.

"What we look at is how do we bring the best version of Tomb Raider to market? How do we actually make it as prominent as it can be? And there's no doubt that having a first-party in Xbox, alongside, saying, 'We want to help you make this great, we want to push this and get behind it, and love it in the same way as you guys do,' there's only benefits that can come from that," he said.

The deal also means Crystal Dynamics can tap into Microsoft's hardware-specific expertise to help with the development of Rise of the Tomb Raider, Gallagher said. From the start, Microsoft employees have shown initiative to help Crystal Dynamics with the project, asking things like "How can we help?" and "What do you need?," Gallagher said.

"It's been a great partnership so far," he added.

Also in the interview, Gallagher reiterated that Rise of the Tomb Raider could come to PlayStation consoles sometime in the future. This is not too surprising because, after all, we know that Microsoft's exclusivity deal for the game has a duration, and it's not forever.

"We're focusing on Xbox One and Xbox 360 here at Crystal, and that's what we're focused on right now," he said. "And anything beyond? We'll come back to at a later date."

Finally, Gallagher confirmed that the 2013 Tomb Raider reboot has now sold 7.5 million copies.

For more on Rise of the Tomb Raider, check out some newly discovered screenshots below. Rise of the Tomb Raider launches this holiday for Xbox 360 and Xbox One.