Why can some people although allergic to peanuts still ingest smooth peanut butter without a reaction?

This is a misconception. There is no such thing as a peanut allergy where a person can safely eat smooth peanut butter but not whole peanuts. If someone is allergic to peanuts, they should avoid all forms of peanuts and peanut-containing foods, including smooth peanut butter. Even a small amount of peanuts can cause a severe allergic reaction (anaphylaxis), which can be life-threatening.