In the meantime since I made this post, I found the source they use for ecoregions:
Ecoregions 2017 ©
With the tree editor tool available on this forum, it is now easier to adjust the biomes for each ecoregion. Now I know what region I am editing 
Really, the main one that stuck out like a sore thumb for me was the Central US forest-grasslands transition EcoRegion. This region covers Southern Wisconsin and Northern Illinois. By default this EcoRegion is set to ConiferousCold Biome. I changed it to a custom Biome that has primarily deciduous type trees.