How does my trivia crack icon change?

This is not an earth shattering question, but I am curious. My Trivia Crack icon on my nook changes. It was the usual trivia crack looking clown icon. Then it changed to a Samsung robot several days ago, then yesterday it was back to the trivia crack guy but with green around it and now today it is back to the robot.

I didn't do anything to it -- just turned on my nook. My daughter laughed at me when I tried looking this up on google - she says it is just my nook not everyone's. But if it is just my nook and I didn't do anything how did this happen?

Yes I am dumb when it comes to this stuff. And my daughter cannot seem to explain it to me even though she seems to expect the fact that this creature mysteriously keeps changing on me.