Adjustment layer icons?

[mjs1973]

What's up with the icons on adjustment layers? About half the time I add an adjustment layer, the icon is the half black, half white circle. The other half of the time there is an actual image of a histogram if I add a levels adjustment, or a curve diagram if I add a curves layer. I also took an image that had the diagrams for icons, and drug those adjustment layers to a new image. The icon changed to the circles on the new image. I tried this again with two other images, and the icons stayed as the diagrams.

Not a big deal, but I'm curious.

Here is an example. I processed both of these two images the exact same way.