Why are the Mazel Cannon balloons showing "Sold Out" when there's still an empty space in the balloon holder?

In the Imagine Museum, an award location for students, there is an exhibit called Mazel Cannon. In the store students can increase the level of 5 different aspects of the gameplay, one of which is the number of balloons the student can hold at a time--or Balloon holders--(top balloon image) and the frequency that balloons appear in the environment (bottom balloon image). You can only increase each aspect so much, as indicated by the power bar to the left of each icon. Once the max is reached, the item says sold out. max_level_reached.png

 

In the image below you can see that although the max level for the number of balloons that can be held at a time has been reached, there is still a space to add another balloon in the holder.

This is because there is a balloon holder hidden in the game as an "Easter egg". This one can't be purchased from the store, students have to find it and collect it. Hiding "Easter Eggs" is a fairly common practice in games. Angry Birds, which this game is very loosely based on, employs such elements frequently.

 

 

As you can see in the pictures below, the extra Balloon holder is hidden just off screen behind the cannon. Students have to use a turn to push themselves backward and try and collect the balloon holder.