I made two resolved maps of the entire surface of the reference sample (200micron scan step) where the surface is behind the pump (flipped sample). One scanning along X axis (left in the image) and one scanning along Y axis. It shows that there are little fluctuations and they are a bit larger perpendicular to the scan axis. This happens for both maps. This means there are some long time power fluctuations. Maybe it shows more clear in the second attached image where there is only  a 2D map.

There is also a relevant difference between the center of the sample and near the edge, the maximum is near the center, and the difference depends on the map. Still to be understood.