This is sort of a head spinner. I am not saying we do anything that has to do with the size of the ungrounded conductors with the 60º table.
Here is my potential argument a little more comprehensively:
The ground is installed to carry enough current to hopefully trip the breaker in a fault condition. That means the code section is concerned with the circular area of the conductors not
the insulation properties. As such there is not reason we shouldn't be able to use the most friendly information regarding the conductors that is available.
I don't know how they came up with values in 250.122, but would assume they tried to find values that are effective in some worst case situations.
Reality is the amount of current that flows depends on how much current the source can deliver and the impedance of the circuit. The trip clearing time depends on mechanical limitations of the overcurrent device, intentional built in delays in some instances, as well as the amount of current flowing.
We could size EGC's with complicated calculations or we could use a one size fits all table like we have. There is probably many cases where the sizes in 250.122 are larger then necessary for effective fault clearing before we even get into this area of increased ungrounded conductor size.
I have already witnessed cases with long circuit conductors that even if you have same size EGC as ungrounded conductors it is obvious the resistance of the long circuit does limit fault current enough that trip times are definitely longer then for short runs.
We must also remember general branch circuit and feeder overcurrent protective devices are there to protect the conductors if you want other equipment protected you may need special considerations in the trip curve of the device used then what is the minimum requirement per NEC.
