Speed= distance travelled / time taken.
Since A and B have both travelled 8km (vertical axis) in 8 hours (horizontal axis) there average speed over the 8 hours is identical.
However
Since at any given moment in time, velocity = distance/ time where the unit of time is approaching zero.
This means that velocity at any given point is given by the GRADIENT of the tangent to the curve at that point.
Your B curve is linear so the gradient is always the same. Velocity=speed= 8/8 = 1km/hour
The A pace changes.
Perhaps you can tell me what is happening. When is the gradient of the A curve equal, less, or greater than that of the B curve ?