When I use a Selective Color Layer to tune my colors and then merge that layer, the colors shift. This can be seen in the histogram when set on colors as well as in the photo. I work in either Adobe RGB or ProPhoto RGB. The same thing happens with a Shift+Ctrl+Alt+E to make a new layer of it.
I have tried at 100% and it does the same thing. I did not have time to tint a B&W and did not save a PSD of the one I did. But I made a video. I am not sure how it looks on an sRGB monitor, but you can see the color shift when I merge and step back in the histogram.
100% is a significant number. It means one image pixel is represented by exactly one scren pixel.
At any other zoom ratio, you're looking at a downsampled and softened version. Sharp pixel transitions are replaced with blurred intermediate values that aren't really there in the original full data. All adjustment/blending previews are also based on this smaller and softened version.
With noisy images, or binary images (halftone dots, line art etc), this gives a misleading preview. To be clear: the flattened version is the correct one. The preview is wrong.
To avoid this, always check at 100% first, to see the correct effect of the adjustment.
In a "normal" photograph this doesn't matter. It's only an issue when you have sharp pixel transitions; individual pixels that are very different from their neighbors.