Difference between "supposedly" and "supposably"
Supposably does mean "it is possible to suppose", as opposed to the very different meaning of "supposedly"!
Supposably is quite rare in actual use; I believe it's more an inkhorn term than one actually found in the wild, so to speak. Thus, I fear that when you encounter it you are probably really encountering somebody who meant to say "supposedly".
Must be a Southern thing. Almost all the natives here use supposably when they mean supposedly. Drives me crazy. I never heard it before I moved to NC.