calendar - 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?

Answer

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