Why region render with the same settings - become cleaner than whole image?
Different passes number?
No passes are constant!
What kind of global illumination (primary and secondary)?
PT+HD! HD cache(PT samples 512, Max records 9000) MSI20
It depend by HD cache solution, try to save HD solution on full image and then load it for region render.
If you are saying that a smaller region rendered clean faster than the whole frame then this is perfectly normal. 
Maru -read, pls, more attentively) I said that with the same settings region render is cleaner(passes are the same) and of cos faster, its obvius&!
So, did you solve the problem saving hd solution?
Yeah, Thanks btw! But interesting now - how interpolate HD settings on whole image? or it won’t work because of the larger image - more complicated tracing?
Hello VadoZe
what you say is true!
also happens with the crop