Why is ionic strength (mu) calculated differently for these 2 examples?
The third pKa of phosphoric acid (going from disodium to trisodium phosphate) is 12.32.  

When trisodium phosphate is added to water it is a relatively strong base, beginning to titrate H20 to OH-, to a significant degree.  In other words, to be above the pKa of 12.32, the concentration of OH- needs to be at least 0.02M.

In the other examples, since the other two pKas of phosphoric acid are 2.15 and 7.2, the pH in those examples will be about midway between 2.15 and 7.2 for the monosodium salt and about midway between 7.2 and 12.32 for the disodium salt, so the concentrations of H+ and OH- are insignificant compare to the other ions.