GMAT Test Prep: CR-42371230 GMAT Critical Reasoning

A milepost on the towpath read"21"on the side facing the hiker as she approached it and"23"on its back.She reasoned that the next milepost forward on the path would indicate that she was halfway between one end of the path and the other. However, the milepost one mile further on read"20"facing her and"24"behind.

Which of the following, if true, would explain the discrepancy described above?
Select one of the following answer choices