I don't know why either. I basically did what you wrote, and the results were not as expected.
There's a couple of mitigating factors, I was printing thin walls vertically because my bigger piece had vertical thin walls, and maybe the filament was damp. The PETG has been in a tight bag with silica gel for a while now. I'll try again in a bit.
Unfortunately when I printed a thin structure test cube in vase mode, the 40 x 40mm base sort of fused to the sheet. Can't get rid of all of it. Most of it is gone, but it's maybe 0.25mm higher in the dead middle of the satin sheet. Can I use a metal scraper to get it off? Tried a pla scraper but it didn't take off much.