Goodwill impairment

Hello dear tutor,

Would you please help me about impairment of goodwill when we have calculate the goodwill with full goodwill method. I don’t understand why we allocate the goodwill impairment expense to both parent and NCI based on their investment proportion and not their REAL proportion on goodwill!!

For example suppose we have :
Fair value on subsidiary net asset = 100
Parent (80%) = 150
NCI (20%) = 21

So in this case the goodwill will be 150+21 – 100 = 71, ( so we know 70 of the goodwill is for parent and 1 for NCI)

Now suppose goodwill has to be impaired by 10%:
so the impairment loss = 71*10% = 7.1.

According to my logic: we have to deduct 7 from RE (ie 70 * 10%) and 0.1 from NCI (ie 1 * 10 %).

But according to the method of the book we have to deduct 7.1 * 80% from RE and 7.1 * 20% from NC!!! WHY? why we are using their investment proportion and not their proportion on goodwill! This method does not seem logical for me at all 🙁

Thank you for your kind help