Problem with feather tool

I select an area with the rectangular marquee tool and then select "feather". When I click ok, the are to be feathered does not remain rectangular, but instead takes on a dome shape that varies depending on the size of the area I selected and the pixels to be feathered.

I have NO idea why this is happening, as I've used the feather tool before and it has just feathered in a square. I don't want it to feather all around, I just want it to feather left/right. Does this make sense?

Help, please!

If you just want to feather left and right only, make your selection so it touches the very top and bottom of your canvas, then feather. It'll only feather want you want it to...