Primarily because (unless you're postualting a simultaneous L-E fault, which would be stretching things!!) if an excessive current goes through the 'blobs' of the L switch contacts, the same excessive current should go through the 'blobs' of the N switch contacts. If thermal damage is restricted to the L ones, that strongly suggests that there is a specific problem of contact of those contacts - not just an excessive current, per se. Doesn't that make sense?Why do you say that? Surely the little 'blobs' in the switch which make contact are the weakest parts (smallest points of contact under proper usage) of the socket.
Edit - having reread your reply, you seem to be agreeing with me.
Why, then, is this damage inconsistent with my point?
Kind Regards, John