flowerbug
Garden Master
I think part of it may be that the common bean was actually domesticated TWICE, in two different regions (Mesoamerica and the Andean region.) So there is probably a lot of genetic variation between the two groups and so lots of odd combinations that can show up.
i've noticed that some of the crosses between the two groups will struggle to be viable in comparison to others who exhibit hybrid vigor. it can be rather stark.