Thing is, the ray is being absorbed twice and is being total internally reflected once.
So first I is being absorbed so we have 0.39I being transmitted to the bone and muscle boundary. 0.33 times 0.39 is then being internally reflected so we have 0.33 x 0.39I. Then again when it is moving from the boundary to the outside, it is again being absrbed so we have 0.39 x 0.33x 0.39 I which is why they squared the 0.39.
Why 0.33 x 0.39I ?