The initial timetick of log 5 is set to 100 Ka (and not 98 Ka) to align the CGLH Timeline with the standard geologic timescale, which does not distinguish between A.D. and B.C. and thus adds 2 Ka to the scale throughout. Take the Neolithic Revolution for example: it would be dated 10,000 "Before Present" (BP) and not 8000 B.C. in the geologic timescale.

By the way: the datum for the geologic present is 1950 in accordance with the convention in radiocarbon dating.