'those for' vs 'those of'

Hi there. I came across a sentence which went like this:
[T]his can mean that the verbal skills of bilinguals in each language are generally weaker than those for monolingual speakers of each langauge.
My question is: why use 'those FOR monolingual' ??? rather than 'OF'
Cus my understanding is that ''..than those for monolingual...' can be re-written as: 'than the verbal skills OF monolingual...'
Thanks!