I presume there could be two reasons:
- the report built in the past did not include all data due to latency (for example, if the report was run on 1 Mar, not all of the data for 28-29 Feb could be fully processed at that time)
- the data for the past period was onboarded (uploaded) via Data Sources after that report was delivered
Also double check that you are working with a regular report suite, not with a virtual report suite as the latter is impacted by the applied to it segment.