Papers presenting results that are worse than random chance
Not all probabilities are 50/50.

I'm assuming that the paper you are looking at is eYOLOv3-Lite: A Lightweight Crack Detection Network.

Under the evaluation metrics section, I see 


  For each image, the intersection over union ($I_{oU}$) between the bounding box of the detected crack
  and ground truth can be calculated as: $I_{oU}$ = $\frac{A_o}{A_u}$
  , where $I_{oU}$ is the intersection over union, $A_o$ is the
  area of overlap, and $A_u$ is the area of union.
  When the $I_{oU}$ of the predicted bounding box and ground truth is greater than a certain threshold
  value (e.g., 0.5), it is considered to be a true positive; otherwise, it is a false positive.


It looks like the problem is to assign a bounding box to each image such that the bounding box contains the crack (or the ground truth box associated with the crack). It's not an easy probability to calculate exactly, but it's quite clear that assigning a bounding box completely at random will yield a success much lower than 50%.