Merge Down/Flatten not giving expected result

[terrypin]

I have two layers in my PSP 8 image as shown. Both are identical except for the shading on the respective Start/End captions, which are either yellow or white. When I Merge Down or Merge All or Flatten, I expect to see both the Start and End captions shaded yellow, my required result.

But only one of them is, the other remains white. And, to my surprise, this remains the case even if I reverse the positions of the two layers. What's my mistake here please?



Of course, I can get the result I want by other means (such as simply copy/pasting a selection) but I'm curious now to discover my misunderstanding.

Edit: A minute later the penny dropped. But I'll leave the post in case it's of interest as an example of wrong-headed logic!

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Terry, East Grinstead, UK