Ha..good question really. I don't know the details of the culture/tile mechanics, but I do know they get a bit more complicated or better convoluted with sea tiles. It appears that you have two Greek cities possibly culture pressing that particular tile. The Greeks must be really emphasizing culture at this stage, and have been for quite some time, to overtake that tile. Otherwise, I really don't have much to explain to you on my end. If you want to win by culture, then you should be in full culture mode. Doing so, should shortly get you back that tile as well assuming you have culture modifiers in Bombay. I would check the victory screen though to see where the top AIs stand in terms of achieving a victory. Like the Greeks may be very close to culture win, unless another AI has gone culture and doing better than them.