operating asset beta getting, understand, change, because

Chuckrox8

Ok, just getting to the end of SS# 5 and I can't for the life of me figure this out. First real point of frustration so far so I guess thats good.

When you calculate operating asset beta before pension assets, why would it change later?
I understand total asset beta will change because you are bringing in the pension assets, but I don't understand how operating assets change when their value doesn't change. To me it seems that the only thing that should change is their weighting in calculating total asset beta.

Any help with this?