Why are some meetings in a frame, and others with a plain background? Some meetings in Google Agenda, coming from the same agenda, are in a colored frame, while others are with a solid background:



What is the reason for the difference? Is there a way to fix this to have unified colors?
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