I don't know, still looks like toning to me. I've seen that sort of toning pattern on plenty of 40% halves. As to "why", it might be uneven mixing of the alloy, but more likely the pattern of stuff the coin was exposed to, or of stuff on the coin that kept parts of it from being exposed. Someone might even have tried to clean it.
Looks like it was worse and someone tried to clean it. See the collection of dirt around the devices.
Agree with the Inspector, many tiny nicks all over the highest devices + noted crud. At first I thought it was cardboard toning.
