Addition Amplifier equation doesn't seem to work
Now you have provided the supply voltages, it seems I was correct in my comment. You say you have a supply rail of +/- 15V. Let us look at the DATASHEET for the 741 Op-Amp: 



You can see that the output will only get to a maximum of +/- 14V, which means your -13.04V that you measured is within its maximum output (depending on load). 

Page 7 of the datasheet shows a diagram of the 741 internal schematic. As you can see, it is mainly resistors and transistors, these will have losses over them, which is why the op amp can never output all the way to its supply rail.