Unity uses quaternions in the rotation but the actual animation curves are still interpolating Euler angles. Same with Unreal, Maya, 3DSMax, Blender etc... GLTF

Unity uses quaternions in the rotation but the actual animation curves are still interpolating Euler angles. Same with Unreal, Maya, 3DSMax, Blender etc... GLTF requires you to convert the animation curves to quaternions. That's a lossy operation