BPP Book illustration 2 Defined benifit plan,pg 87

Deat Richard,

in this ilustration past service cost i,e gain on curtailment is calculated as 12-9=3 leading to decrease in pension plan liability by 3 only,,,, why is 9 ,the benefit payment deducted to decrease the liability where this benefit payment has net effect i,e it reduces both pension liability ans asset i,e
Dr pension plan liability 9
cr pension plan asset 9
further more the reduction in employees lead to reduction in liability leading to a gain i,e
Dr, pension plan liability 12
cr P/L Gain 12
thus the pension plan obligation is decreased by 12 plus 9 so why 3 is calculated?,,
plz explain