I've got an RGB image, but each pixel actually has identical R, G, and B values-- the image is just varying degrees of grey. (It's not actually a greyscale image, long story. :P)

If I click on the single channel view, it's much brighter than the RGB combined view. Each channel appears identical to each other channel, but they're all considerably brigher than the combined RGB image.

Do you know what adjustment is being performed?

(That's the short question; elaboration follows.)

Photoshop is aware of this adjustment, and will replicate it if you copy the RGB image and paste it into a single channel of a new image. For example, I can copy the entire image, create a new image, and then paste into just the red channel of the image. Visually, the combined RGB view of the source image and the single channel view of the dest image will look identical.

How was this achieved? Photoshop darkened all of the values: a pixel that was 24, 24, 24 in the source image is now 7 in the new red channel.

I haven't been able to replicate the conversion. Visually, it looks a lot like a gamma adjustment, and values in the middle range roughly approximate an adjustment using a gamma of 1.266 (which seems odd, so is likely not what's occurring). Values in the lower range, however, are darkened further-- a gamma adjustment of 1.266 will leave them too bright.

Photoshop's channel viewing on screen is governed by the grayscale profile you have loaded. Changing that profile will change how those channels are viewed. If your grayscale profile as loaded in Color Settings has the same gamma as your RGB working space, the individual channels should view the same as the composite.