Accuracy of location from pings.
My understanding is that "width" of the circle on the earth corresponding to timing/distance measurements (satellite to aircraft) could be very small. Satellite messages often have very accurate clocks on each end.
The accuracy of limits of the gap area, the missing arc over SE asia under POR coverage, would be much less. With POR at a very low altitude above the horizon the possibilities for signal transmission may be affected by the surrounding landscape.
Eg, you may be in a valley in Vietnam and POR is above the hypothetical 0 degree horizon but is obscured by a mountain.