Either because you only have 1 month of raw data retained or because the timerange for your report only spans 1 month. Acceleration (or lack thereof) cannot ever cause a search to return different results; it can only effect how quickly the (same) results are returned.
OK. Thanks. My understanding of report acceleration must be wrong - I thought report acceleration actually took the data from your search, and put it in "side buckets", allowing you to keep longer term storage for a report, without having to keep the detailed data. Is that wrong?