Aloha tribu,
You're not in a different "zone." Sometimes the wiring, etc. in an office or home can mess with your compass. Check it out with a friends compas, your's may be broke or damaged.
Marie mentions to do the measurements during the day with the lights, computers, etc. off, including not having any metal on you, and taking a shower/bath with lavendar soap for good measure.
Standing in certian doorways can throw it off too - so she has one take a measurment two feet in and out of the door way, just to make sure things are on the up and up.
Much Love, Shawn