There are two exceptions -- one is if it's an express service, just 30 minutes instead of the usual 50 minutes or an hour. In that case the massage therapist might concentrate on one area, usually back, neck and shoulders, so you get more benefit.
The other is when you ask the therapist to concentrate on a specific area during the full-length massage. They take more time on that area, and usually skip or go very lightly over other areas.
Unfortunately, some "spas" that operate like the old "massage parlors" put out signs that say "full-body massage" as a way of indicate there will be some kind of sexual contact or "happy ending." This is illegal and an unethical use of the term full-body massage.