All of the above. Poor maintenance is probably the biggest reason but you have to remember who the user was and factor in the region and climate it was used in. They were soldiers using a tool. The rust pitted bolt or carrier that didn't quite survive to end up in a show case more than likely saw use in the worst conditions or was used in training.Damn those bolt scrubbers!
On a side note it seems G3 bolt heads like to rust or pit more often than other surplus bolts I see.
Is it because of white metal / lack of a rust delaying surface finish or just poor maintenance on the users part?