When mixing tree groups, the forest generator only respects the group weights at a cluster strength of 0.
As the cluster strength is increased, the resulting mix will start deviating from the intended ratio.
Reproduction:
Preparation:
Create a simple world for test purposes, in there create a new (non prefab) ForestGeneratorEntity on a polyline of somewhat large size.
Configure the ForestGeneratorEntity to only have a top level, which contains 2 tree groups, which each has only a single tree (e. g. picea_abies_3f and pinus_silvestris_3f, both with the exact same settings!)
Set the weight of group A to 0.1 and the weight of group B to 0.9.
Now, change the cluster strength to different values, regenerate the forest and observe the result. Leave cluster radius at 15 for simplicity.
The "expected" result would be that no matter what, the ratio between tree types would very approximately follow the 0.1-0.9 ratio.
That is in fact the case when using a cluster strength of 0, but when increasing it, there will be big changes in the resulting mix of trees.
Additional information
As I mentioned, the issue can be measured before SelectTreeToSpawn(), so it is likely related to the actual group selection.
Whoever investigates this is welcome to contact me (tilw) on Discord to discuss the topic further.