Did you ever think about what those magnets are actually grabbing onto? On lath and plaster the nails holding the wood strips to the studs are what pull the magnet, so the reading runs a bit off center from the stud itself. With nails it reads fine, but that's why some folks get fooled on old houses where the lath was nailed crooked or doubled up in spots. You still end up close enough for hanging a shelf or a picture, just worth knowing the magnet is following metal, not wood.

Reminds me of a buddy who swore by cheap painter's tape for masking baseboards. Did a whole living room that way, felt real proud of himself. Two days later the tape peeled half the finish off the trim and he was out there with a razor blade and a touch up brush. Some corners are just not worth cutting, even when you think you know better. Man was real quiet about it too, which for him is saying something.