How most trainers measure their clients is one of the dirty little secrets of the health and fitness world.
Most of them try to cheat you.
How can a trainer cheat you with measurements?
Many trainers try to lull you into a false sense of progress by measuring you in dozens of different sites (2 on the neck, shoulders, 3 on the torso, 3 arm sites - both arms, 5 leg sites - both legs, etc.). When you see a BIG number at the end of several weeks of training, crummy trainers have an easier time tricking you into buying another package.
"Wouldn't you like to continue seeing numbers like these, Mrs. Jones?"
GPP only measures in 7 sites. Most of the time 4. It is enough to see progress, or lack thereof - which is the only purpose for measuring.