You're right, that does seem strange. I'm seeing what you describe as well, and it does seem at odds with the datasheet. However I just looked and there's no inversion done in Espruino - those numbers are as they come from the sensor
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
You're right, that does seem strange. I'm seeing what you describe as well, and it does seem at odds with the datasheet. However I just looked and there's no inversion done in Espruino - those numbers are as they come from the sensor