Here's how it works for a skirt offset of zero
- If "Generate support" is disabled, then it does exactly what you want. The skirt would touch the edges of the bottom layer creating what people seem to call a "brim"
- If generate support is enabled then the outline for the skirt does what jimc mentioned. It uses the silhouette of the part, which may or may not match the bottom layer outline. It depends on the geometry of your part.
I think this makes sense, because if you are using support material, you don't want the skirt interfering with the base layer of your supports. That first layer for your support structures is really important, and you don't want the skirt messing it up.