While on a hike, a person decides to hike from Point A to Point C, via Point B. The signs on this particular trail head describe trail length in time, not distance. The sign at Point A says that the average time to hike from Point A to Point C via Point B is 8 hours. The hiker is fine with this arrangement, so he takes off.

The hiker arrives at Point B and eats a snack and rests for a few minutes. Refreshed, the hiker references the sign post at Point B, and notices that the time listed to both Point A and Point C is three hours. The hiker also notes that it took five hours to hike from Point A to Point B. The hiker shrugs off the confusion, and continues on to Point C. Sure enough, the sign was correct and the hiker arrived at Point C approximately three hours later. How are both sign posts at Point A and Point B correct?

The path from A to B is uphill, so it takes longer to hike from A to B than from B to A.