They believed Superman Returns would do 500 Million WW, not domestically, and Alan Horn was quoted as saying so?
Hardly seems arbitrary?
Again, that is not even close to Spiderman numbers. If you want to look at this domestically you could cut it down the middle to make it easier and say maybe WB was looking for Superman Returns to make 250 million domestic, again not even close to being unreasonable. On your chart this would fall about 90million below Spiderman 3 domestically, the worst performer out of all the Spiderman movies.
So again how is 500 million WW even close to be unrealistic for a Superman movie? Even if this was true, which there is nothing you have said that proves to me it isn't, Superman Returns didn't make back it's budget, it is right there in black and white. So although it wasn't a failure, it wasn't a profitable entity against it's budget. Which is what all studios look for.
That being said it is still getting a sequel.