Comparing date ranges is a nice tool in Omniture, but it's missing a vital feature: the report is sorted by one of the two date ranges, the report returns results only for the sorted date range. For example, run a Pages report for last month and last month YAG. If report is sorted by last month YAG, only the pages that existed last year will show up. New pages, that had page views last month but not last year, will not display unless you sort by the newer date range. Similarly, pages that were expired this year (had page views last year but not this year) will not show up when sorting by the newer date range.
I would expect results to be returned for both sets of date ranges, even if the line item has 0 page views for one of the date ranges.