What's going on is that you're not thinking.

In real life you can't fill a terrain with 100% of trees and then put something more on it. If you have 100% of something anywhere, that's it. Can you have 125% of your salary? You have 100% of your salary.

In real life, you can have a terrain with for example 75% trees and 25% ferns. That's normal life.

If you increase the ferns to 30%, obviously the trees go down to 70%.

If you want to decrease or increase the overall density, you have a different slider.

You have to think.