The section on the death toll seems a bit confusing to me. It says that over 600,000 people were sentenced to death, but only 200,000 were executed. Does this mean that two-thirds of people avoided execution? That seems very high and unlikely to me.
According to the book, Stalin set a limit of 72,950 executions. Some local authorities passed more death sentences, but many of those were for people who had never been arrested. Stalin realized what was happening and pardoned many people before they were executed. Only 300,000 people were ever arrested during this period.