I think it's because it wasn't underlined, or in bold, or somesuch. ;-)

p.s. The reason why this matters is because some people, like myself, use CSS to show sigs much lighter than the normal node text, because I find sigs distracting. So if you put your whole text in your "sig", it'll all show up lighter — almost unreadable, and very distracting, to read.

Personally I find the fact that the sig in the same textarea as the node when editing, a design mistake on this site.