I agree, the only reason I filtered down to posts is because my query has to cross all comments against each other. As a result it was too large a dataset and the query wasn't finishing. Applying some limitations allows me to run the query and get some meaningful results, even if it would be preferable to have commenters included as still active.

But on the positive side, by using top level comments (ie. posts) as the standard for both months, it is using just one standard of what can be considered 'active'.