I have found that moving my detector even an inch or so will change the average CPM. I t may be due to orientation but also how close it is to any background source. Wood, Granite, etc... I have mine presently on a wood stand and it's been consistent for three months. I get gradual variations that increase or decrease the counts by 2-3CPM depending on the weather and who knows what else! I think the most important thing is to mount it in one orientation and not touch it. That is the only way to adequately gauge the changes over time. None of these small detectors are going to be an absolute reference detector. We can only determine an approximate increase or decrease in count and not much more.