Apparently Mass Effect 2 on the PS3 is utilises the updated engine Bioware are using for Mass Effect 3, reports Eurogamer.
"We actually created the engine for Mass Effect 3 and used that to make Mass Effect 2 PS3," claims Jesse Houston on the Bioware podcast. "So we took the content, the story and all of the other assets that made up Mass Effect 2 and we put it into the Mass Effect 3 engine."
So there you go - another mystery solved.
PS3 Mass Effect 2 uses ME3 engine [Eurogamer]