With an object height of 17mm and a step down of 3mm Kiri:Moto seems to figure out that 6 passes of an actual step down of 2.84mm need to be generated. So far so good.
But when Z Bottom is defined to e.g. 1mm, the same step down of 2.84mm is used and only the last pass is dropped. This leaves 2.84mm of material instead of 1mm.
Not sure if this is a bug. Maybe the Z Bottom should have been part of the initial step down calculation.