Good video accompanying your question and this answer is based on both the question and the video.
You need to carefully read the user manual. It appears from the video that you either have not, or are ignoring significant parts of it.
Specifically, pages 20 and 21 tell you exactly how to hold the device and what to expect if you do not, or if you (for example) hold it with a stick. Page 22 further describes techniques for detecting live cables: detection improved by cables in use, by moisture in walls, by what you wear on your hands and feet, and by the positioning of your hands and feet. Page 23 explains how to recalibrate the instrument if your readings seem suspicious and pages 24 and 25 explain how to diagnose problems such as yours.
The instructions are, as far as instruction manuals go, freakishly thorough, clear, unambiguous, and helpful. I'm certain that if you read and follow all the instructions carefully you'll be successful.