I've seen some inconsistencies as well between data available to the Graph and what is actually seen via the Facebook UI.
However, if you can live with relying on the results from the Graph API, you do have some ways of getting the information you're after. If it involved pagination of results, you may be disappointed in the results.
You can run FQL thru the graph API (and even from the Graph API Explorer). Try
fql?q=SELECT post_id, comments, message FROM stream WHERE source_id=me() AND created_time > 1326064184 AND created_time < 1326634407
From this query you get the stream items for the user as well as the comments object for each of those posts.
If you are looking to pull comments counts for a time period on the user's posts, then you can use: fql?q=SELECT object_id, text, time FROM comment WHERE post_id IN (SELECT post_id FROM stream WHERE source_id=me()) AND time > 1326064184 AND time < 1326634407