How come the amount of created tickets in reports is smaller than the sum of resolved and unresolved tickets?

The number of tickets created during a period of time refers to only the new tickets that were created during the time period. But the number of resolved, unresolved tickets count takes into account the tickets that were created already during an earlier time period but which are resolved and those which remains unresolved during the time period for which the data is filtered. These tickets might not necessarily be created within the date range but can be created at any point in time earlier.