Does Over combine both FG and BG alphas?

Hello,

I'm having trouble understanding some strange (perhaps normal) behavior of the Over node. When I layer two of shakes Image creation nodes (eg. Color and an RGrad), each with their alphas set to something other than 1, and the Over node set to preMultiply, the resultant image's alpha channel seems to be a combination of both the FG and BG's alpha. That's strange because I had assumed that the resultant image would contain the foreground's alpha. I also thought that only the foreground images alpha was considered in the composite, i.e.:

Over= fg*fg_Alpha + bg*Inverted_fg_Alpha

The problem goes away though when I do the same operation using a layerX node set to use the above equation instead of an Over. In other words, the layerX node layers the two images according to the above formula, and then uses the forground's alpha as its alpha, while an Over node layers the two images, but combines the alphas of the FG and BG for its own.

Hopefully I worded that well enough not to cause too much confusion.

Thanks,

-Tyler