blast wave reflection vs angle of incidence

huangyhg

blast wave reflection vs angle of incidence
i'm trying to understand a graph in the publication "design of blast resistant buildings in petrochemical facilities" published by asce.
on page 3-7 fig 3.2a shows the relationship described in the subject of this post for a shock wave.  i fail to understand why the reflection coefficient cr reaches a peak at an angle other than zero.  for example ps/po=0.136 has a peak cr at an angle of incidence of about 67 degrees.
can anyone explain why this is the case?
thank you