Adjective placed before a noun but adjectival complement after it

Page 14 of Practical English Usage reads

In some cases an adjective can be put before a noun and its complement
after it. This happens with different, similar, the same, next, last,
first, second etc; comparatives and superlatives; and a few other
adjectives like difficult and easy

The best mother in the world

Why are the adjective difficult/easy dealt with as being different form different/similar?

Secondly, what other arrangement of words would be grammatical for superlatives, as in The best mother in the world ?