Human body (or skin) resistance is very nonlinear and varies a lot with voltage, moisture, time of the day, person-to-person, etc. This Wikipedia article provides some information on that:
https://en.wikipedia.org/wiki/Electric_shockI don't know though why a value of 1.5 kOhm and not 1.0 kOhm or 2.0 kOhm was selected for HBM model.
Max