The meaning of 'I don't suppose so' is simply "No." I suppose so = I reluctantly agree, I guess so, as in "Do you think it's going to rain?-I suppose so." 'I don't suppose so.' = 'I suppose not' = I guess not. I don't think so.; I expect not.; I suspect not.; I think not. A vague statement of negation. (More polite or gentle than simply saying no.) Ref:(http://idioms.thefreedictionary.com/I+suppose+not)

Yes you are correct. Other examples could be: A: "Are you happy that our local soccer team won their game yesterday?" B: "I suppose so" ~ Person B is saying 'I suppose so' in answer to person A since they don't really care too much that the team won since its not in there interest, but it can be better than saying 'I don't care' since that might seem a little uncaring or rude.

It is like" I don't know".