Change in Yaw when Roll is changed While using Mahony lib

I am using https://github.com/PaulStoffregen/MahonyAHRS which claims to give intrinsic rotation values of 101

https://github.com/PaulStoffregen/MahonyAHRS/issues/2 here i have shown that change in roll(with respect to y axis) changes yaw as well as pitch. which i am not able to comprehend.

Can any one please help.