@zmp2000 I'm pretty sure it is because the ALH is decreasing the height of more layer than it is increasing it. You have a base layer height of .1mm, with ±.2mm variance (the first of the ALH parameters), witch mean that you can go from 0mm (or whatever is the closest to 0 layer hight) to .3mm. You can easily control it by using "layer view" -> "layer thickness".