Hi @neha8 ,
Although there could be many reasons for your smart list data not matching the report data the two main reasons i think could be:
Reason 1: Smart list looks for the data at the real time so if there are any leads which have been deleted or merged between the program run & smart list creation then those leads will not be reflected in your smart list, but will still be captured in the Email performance report.
Reason 2: Being a high volume activity Email send & Delivered activities are only stored for 90 days & as smart list reference data from the activity log at the run time all leads beyond 90 days limit will not be captured in your smart list, but will definitely be referenced in your report.
Hope this helps..