The agent based data is calculated from related purepath, and the data is only calculated from part of the purepath which is only related to this agent.
Let me explain it with couple of snapshot.
The first snapshot: You can get Response time (Average, Minimum...) from the detail of the agent.
After drill down to the purepath, we will calculate the data from each purepath.
From the second snapshot to the fifth one: You can see the data with red line. These data is not from the whole purepath, but from the data only related to this agent.
You can try to SUM the four response time from each purepath, you will get the result in "Agent Details".