8 missiles 9 hits

This is more of question.
Occasionally I've seen that missiles in AA engagement get bonus pK rolls, sometimes. I thought this is a bug.
Yesterday I came across a save which produced this consistently.
A rule??
It seems like

• if a missile group is attacking an air group with N units
• then each missile above 2N gets two pK rolls

E.g. load attached save file, if CVA attacks WUA 2 x Mig-23 with

• 4 missiles > 4 pK rolls
• 5 missiles > 6 pK rolls
• 6 missiles > 8 pK rolls
• etc.

This is how you get 9 hits with 8 missiles:

107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 81
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 27
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 38
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 57
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 14
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 54
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 80
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 76
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 26
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 37
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 48
107161 combat3.c:1519 - AirToAirResolution Ph: 60, Roll: 58
107161 effect4.c:5226 - CheckAirMissileHits 8 x AIM-9R Sidewinder against RED 2 x MiG-23ML Flogger G, 9 hits

Looks like a rule with a history . Care to tell?
Is this from the Paper rules?
It would be more natural to give a probability bonus than rolls, but it's a safe way to double pK indeed.
And why after 2N and not from 2N?
Is it saying that pilots have an ok chance to evade 1 or 2 missiles but have trouble evading 3?

I hoped that this bonus is triggered for WUA because CVA attacks them from the blind, but no, the bonus is granted for head on attacks too.

Are you aware of any other hidden gems of air to air engagements?