I tend to agree with brewtownbeatdown and pbe gummi bear statements. Combination of sun/uv exposure and cooking heat on thick orange peel black porcelain. Something in the color pigment or chemical makeup on the final coat may contribute to fading also.
Here is my A-code I posted on other thread with even fade top to bottom. Don't think Sun/UV exposure alone over time can do this.
As for why not fade inside as asked by Cellar2ful .... I have 2 possible explanation that may or may not apply.
First, due to smoke and grease and over time with film of crud forming on the surface- never get chance to haze or fade. Similar to how applying oil brings back faded exterior to black.
Second is bit more far fetched. When kettle lids and bowls are sprayed at the manufacture, they are first sprayed with black bonder or primer similar but very different to exterior finial black coat which is applied after. I have seen light blue interior on Red MBH Cellar2ful has, but that is another story. This is why every other color lids and bowls has black interior. So my theory is that they go thru. 2 step or 3 step process similar to how automotive manufactures paint their vehicles. Therefore, spray porcelain composition is different for exterior than interior. Concluding that interior spray chemical makeup lacks color pigment or something exterior makeup has on the final coat.