IMU BNO055 - Orientation on y-axis just between 90 and -90

Hello,

I am using a BNO055 to track the orientation of a skateboard. While using the "sensorAPI" example given by the BNO library, which output is the orientation, I am getting 0 to 360° for x and Z axis but just -90 to 90° for Y. Why is it doing that. The problem for me is, that 135° and 45° are the same.. both times 45.
I think i am missing some information. 

Thanks for any help!