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> For example "high mortality" that is created by adding the deaths of Soviet POW. 
Soviet POWs were used as laborers. Those who died in that condition were labor deaths.
That said, there were also deaths among civilian workers and other groups, mostly with the large influx in 1942, for which the coutnry was unprepared; afterwards authorities managed to dramatically reduce their mortality rates.

> Foreign civilian worker, that were all volunteers
Incorrect. There was large-scale conscription of civilians. The roundups in Poland were particularly famous.
> were decent treated
Not at first, but the book explains how treatment greatly improved and they were integrated into the economy with time. Even then, you must consider facts such as the apartheid regime under which Polish workers lived. Take a look at the "Polish decrees".
> received comparable to Germans wages. 
In the Polish case their wage was restricted at a lower than German level.