This is apparently status-bydesign, according to SO employee Aaron Shekey on MSO:
This intentional, but not ideal at the moment. I plan on having a preview card on every user. In my opinion, it's weird UX having it on some users and not all, regardless of a drop shadow or a bio.
I'll update the help article to be more accurate tomorrow. Thanks for pointing it out!
Personally, I'd rather have drop shadows for everyone then. After all, not everywhere you see a username lets you see the card (such as the question list). And not everyone waves the mouse around to see what has a hover.
Also I am not sure how this will work for people that have nothing in their profile to show, because it seems pointless to expand and show no new information.