Writing to a single file is highly unlikely to be processor-bound. Being processor bound meaning that processor resources like CPU is the limiting factor. Such operations are almost always IO-bound. That means being limited by the bandwidth of the storage device.
So increasing processor / cores will not increase throughput for a single file. it might be that 99% seems a bit odd but it might very well be that it uses what extra cpu it needs in one of the other processors but they are hard to pinpoint as it spills over. But I doubt that this is the case here.