IMU coordinate frame

Im a bit confused about my IMU readings. I have the coordinate system set to default, so as i read the documentation the IMU coordinate system should be the same as Image coordinate system. But when i point the camera axis straight down, so the camera system z axis pointing down, then i still read a negative gravity acceleration Z axis from the IMU, shouldnt that be positive when im pointing it down?

i use two zed x one gs in a virtual stereo configuration.

Can you share the code that you are using?