Activity is a list of all activity, including very short chats or unanswered chats. Reports, on the other hand, have some filtering on by default. Those filters are for minimum duration and for including unanswered chats.
Minimum duration defaults to 45 seconds, a number we chose after doing research into what might represent a meaty question. The number of seconds can be changed. This typically avoids counting chats that are very brief conversations by returning guests saying "thanks." This lets the person running the report do their best to avoid over-reporting chat statistics.
To have Reports count all questions recorded in Activity, turn on the Include Not Answered checkbox.