Unicode does not dictate how glyphs are presented. It just describes and categorizes them.
So how they look comes from the font that is used. For the proposal these fonts probably didn't exist yet, so it was probably just a (slightly sloppy) photoshop.
That's a good point, and I should have clarified, I'm referring to the full stars (not half-stars in the new proposal). Not a Unicode issue, but definitely something I've seen at least on macOS machines.
HN strips the characters out from comments, but they're displayed in the beginning of the article.