Zoombinis level ascension

How is stage difficulty determined?

I look into my Zoombini map to be greeted by something strange. Some levels are on "Very Very Hard" difficulty, while others are not even close.

"Titanic Tattooed Toads" and others on that path are on red stage; so how can this be when the Mountains Of Despair are only yellow? What constitutes differences in selection?

Is it the amount played at that path (Who's Bayou, The Mountains Of Despair, etc) that determines the color of one of the path's sections?

I'm kind of against the idea, actually, considering this. Before one and after both campsites, the difficulty is different. Currently "Allergic Cliffs", "Stone Cold Caves", and "Pizza Pass" are hardest difficulty, so you can see that the mediocre difficulty of the Mountains Of Despair is quite disappointing.

Another possibility is number of trips made with the most possible amount of Zoombinis (16). My Zoombinis often collide in BubbleWonder Abyss, so I barely ever get 16 across.

Is my problem

• The amount played on one path

• Flawless completion on my task

Or is this something different?