The result is not the same because:
- in PBR, the fog color is converted to linear, then the final shader (linear) color is mixed with this linear fog color, then this color is converted to gamma space
- in standard mode, the fog color is a gamma color and is mixed with the final shader (gamma) color
So, in PBR there’s a conversion to linear space before doing the computation, then a conversion to gamma space which yields a different result than doing all the computations in gamma space.