The event count in Activities report could be higher than that is present in New user report. New user report shows only the event counts of the segments, who installed your application during the selected timeframe. Whereas, Activities report shows all the event counts of the segments, who triggered those respective events during the selected timeframe.
Example Scenario: When you are looking at last week’s data on device report for all media sources & campaigns. Let’s assume there was an install from Samsung & two events from Samsung and LG respectively, happened as well. Hence for last week, the event count in New user report for device would be 1 whereas in Activities report it would be 2.