SAT Grammar: Tip on Phrases like "the fastest of all land animals"

QUICK TIP

I posted this in an old post:

27. (A) Attaining speeds of (B) up to 60 miles per hour, cheetahs (C) are perhaps the (D) fastest of land animals. (E) No error

Someone asked me if it is wrong to say something like "the cheetah is the fastest of all land animals". Why, the person asked, can we not simply say "the cheetah is the fastest animal?" On the SAT, there is nothing wrong either phrase.

Since most (or all) of you have lived in Taiwan for quite some time now, you may have at some point passed by this store:

PANTS KINGDOM: "Even the biggest of birds can fit in our pants." (That's an owl in there). There's nothing wrong with that sentence. Somebody knows his grammar.