Here the angle of deviation differs depending on the angle of incidence x of the light ray on one of the mirrors. let angle COB = 60 deg = angle between mirrors.
If it is incident at angle x = 90 - ∠COB /2 = 60 deg., then its angle of incidence on mirror 2 is 0. Hence, it retraces the path after reflection from mirror1, then onto mirror 2, and again on mirror 1. So the angle of deviation is 180 deg.
If the angle of incidence x on one mirror is less than 90 - ∠COB /2 ie., < 60 deg., then the light ray is reflected from mirror 1, and then from mirror 2. The light ray goes out. The angle of deviation = 120 deg. (which is actually twice the angle between the mirrors.= 2 * ∠COB )
if the angle of incidence x is more than 90 - ∠COB/2 ie., > 60 deg, then the ray is incident on mirror 2 after reflection from mirror1. Then it is incident again on mirror 1. Then it is reflected back in to air. In this case the angle of deviation seems to be 300 - 2x.