The TBN of Mobil 1 will depend on which ASTM method is used. If you test it using ASTM D-2986, which Blackstone Labs uses, you will see a baseline TBN in the 12.0, +/- 1.0 range. If you test it using ASTM D-4739, which Cleveland Tech Center (CTC) and AVLube use, you will get a baseline TBN in the 10.0, +/- 1.0 range.
In order to properly analyze TBN depletion, you need to use the correct starting point (and the same analysis lab).
I should add that Mobil Delvac 1 (their HD synthetic diesel oil) is advertised as having a TBN of 12.0. Send a sample off to CTC and the TBN will come back within +/- 1.0 point to this value. So the idea that Mobil 1, 5w-30 has a TBN of 15.5 is not reasonable at all.
You can also compare the levels of calcium/magnesium (the active detergent/dispersant additives) for Delvac 1 and Mobil 1 and see that the HD diesel oil has a more robust additive chemistry ....Delvac 1 has about the same level of Calcium, but it also has about 600 ppm of Magnesium. The new Mobil 1, SS formulation has almost no magnesium, maybe 50 ppm or so.