Of course, this will be spoileriffic. I'd suggest you play through the game first. It's on Steam, go buy it, finish it in a couple hours, and come back. Ready?
The protagonist's girlfriend was killed, and the endings were him choosing between killing himself to stay with her, or moving on afterwards. Nothing unexpected there. What's interesting is the circumstances surrounding it: the bombed-out bus, the newspaper clippings about the war, the strange buildings and desert city contrasted with the bright green hills & tree. The kicker for me is that in the green ending, the protagonist looks like he has the tanned skin and black hair of an Arab or Indian.
That's when I realized I made the unconscious assumption this was all taking place in the U.S. Once I moved that to the Middle East, it all became clear:
He met her, or they grew up together, in a foreign land symbolized by the green hills and tree. Later, she returned to her homeland for some unknown reason and is fatally wounded in a bus bombing. The protagonist learns this and rushes to her side, only for her to die as he watches. This drives him insane and he spends several days in psychiatric care, regressed into his own mind, trying to decide whether to kill himself or move on.
Not only is it a psychological twist ending, but it plays off our expectations to make the story seem strange even in retrospect, forcing us to adjust our view to make sense of it.
The more I think about it, the more impressed I am. Small twist makes big difference.