My explanation ran a bit long, but the key thing is that a layer's opacity 
setting is generally independent of the layer's blending.  AFAIK* "Dissolve" is 
the only layer blending that actually uses the opacity setting in its blending; 
for everything else, the layer blending is performed first and the opacity 
setting just interpolates between that intermediate result and transparency.  
(* - Let me know if I missed anything.)

I tried taking a look at your XCF directly, however I'm running stable GIMP 
(2.8) and GIMP 2.9 makes breaking changes to the XCF format, so that's out of 
my range.  And while I was able to reproduce the left half of your screenshot 
easily in GIMP 2.8, simply stuffing those layers into groups did not change the 
result any, leaving me unable to reproduce the right half of your screenshot 
(without further info).

Here's an idea you can try -- move your three layers around so they only 
partially overlap each other (like a Venn diagram).  Then you have the ability 
to see the final blended result plus each layer individually.

-- Stratadrake
Numbers may not lie, but neither do they tell the whole truth.

> Subject: Re: [Gimp-user] Layer addition - bug, feature, or user 
> misunderstanding?
> To:;
> From:
> Date: Tue, 13 Oct 2015 11:46:11 -0400
> On 10/13/2015 10:55 AM, Elle Stone wrote:
> > So does this seem like a bug in the Addition blend mode? Or is there
> > something I'm not still not understanding about how Addition layer blend
> > mode is supposed to work?
> Hmm, Krita produces the results I expected, that is, the three layers 
> add up to R=G=B=1.0f.
> So either GIMP intentionally uses a different algorithm for addition 
> blend mode. Or else there's a bug in the code, most likely the same bug 
> reported here: I'll 
> add a comment to that bug report.
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