In this case the spec is outdated -- the current behavior is intentional after discussion with sgabriel along the way during the top chrome MD work.

I don't know if we need to keep updated specs sitting around somewhere to reflect these sorts of changes?

What was the rationale for leaving it in the old style?

We actually changed the style to this -- it used to be implemented as you see in the mock.

The color change was for better contrast and better alignment with system color schemes.  The width change is more aesthetic; the new selection shape mimics the shape of the omnibox and looks less odd (IMO).  I think ainslie@ has said he'd like to move away from the full-width dropdown entirely in the future given that search term replacement didn't ship (which was the big driver of this design change originally).