Don't understand how Post Process Priority property works

I have three pp volumes red, green and blue which represent atmospheres of the three planets. All three volumes are set unbound. The camera is located on the planet surface with the blue atmosphere. The red atmosphere is farthest from the camera. I’d like that the atmospheres are rendered in the following order: 1 - red, 2 - green, 3 - blue. For that purpose I set pp volumes priorities as such: red - 20, green - 10, blue - 0. These settings yield the following rendering order: 1 - green (10), 2 - red (20), 3 - blue (0). If I set priority of 30 (or any value more than 20) for the green volume then I obtain correct rendering order: 1 - red (20), 2 - green (30), 3 - blue (0). Also, If I set back the priority value for the green volume and disable the blue volume then the rendering order becomes to this: 1 - red (20), 2 - green (10). This behavior looks rather strange for me. Perhaps there is some additional rule that determines the order of rendering according to priority values or is it a bug?