Photoshop :: Reverse Gradient Effect...

I was working on a design. I had a dark gray background and was going to do a soft white gradient effect in my selection.

When I went to do it, for some reason it was reversed. The gradient came into the selection but just above the selection was a darker color. So it looked like a gradient with a shadow all at once.

Then I looked at my layers and noticed I was not in the new layer, but I was on my background layer, so it was one layer.

So when I did the design, I went back to play with gradients and it wouldn't happen anymore. Does anyone know how this happened, it was a nice effect.

My mode was on "normal" and the only difference is that I was on a background layer already filled with color, so I thought that was it, but apparently not.

it looked pretty clean to get a gradient with a shadow effect on the outside of the selection. The only other way it does some of this effect, is when I feather the selection and use foreground/background color with the gradient tool, but it's not the same.