Why are some meetings in a frame, and others with a plain background?

Google Calendar Help says

Invitation status

Events on your calendar look different depending on how you’ve responded:

• If you’re attending an event, the event will be a solid color.




• If you’ve answered maybe to an event, you’ll see diagonal lines
  across it.




• If you haven’t replied yet, you’ll just see the event’s outline.




• If you said no, you’ll only see the outline and the event will be
  crossed out.

So, I guess it's the outcome of how you responded to them