It appears that Tomb Raider developers within Square Enix have stated during a AMA on reddit that the newest reboot of Tomb Raider was built from the ground up on the Xbox 360 platform. It was later ported to the PlayStation 3 and PC versions. This was a question that was posed to Global Brand Director, Karl Stewart and Creative Director Noah Hughes from Square Enix. The question has since been deleted but this was the answer they provided.
The development of the game has really been kept to a very tight group of internal studios. The entire game has been developed and built from the ground up on the Xbox platform at Crystal, Nixxes (a long time partner of the studio from way back in the Kain days) built the PS3 and PC versions based off of the code we developed at the studio, and then Eidos Montreal built the Multiplayer.
Interesting, does this mean that the PlayStation 3 and PC versions of the game are the weakest link? Could also explain why there have been reports of the PC version being patched twice so far in one month since release.
Source: PlayStation Lifestyle