What does this the DH 2e’s Mountain Warrior’s ‘each advance’ mean?


Mounted Warrior

Requirements: Rank 2 (Trained) in any Operate skill or Rank> 2(Trained) in Survival skill, BS 30 or WS 30

Apptitude 1 Weapon Skill / Ballistic Skill

Aptitude 2 Offence

Reduces any penalty for making attacks (Melee or Ranged) from a moving vehicle or mount by 10 for each advance.

I’ve not been able to other sources use similar wordings. Does this mean each rank in all operate and survivor skills gives a +10? Is it just the highest ranked of the 4 skills? Or is it something else?