Problem with combine Rotator ?

I am working with combine rotators and getting a bit confused. I read some articles/posts and from my understanding it works like rotate the first angle, then rotate by the second. So i used in the python API as follows:

fi_pos = unreal.Rotator(0,0,180)    
se_pos = unreal.Rotator(-35, 0,0)
po = unreal.Rotator.combine(fi_pos,se_pos)

But the output I am getting is -

<Struct ‘Rotator’ (0x00000169852961A0) {pitch: -0.000007, yaw: -179.999985, roll: 34.999996}>

The roll is rotated in the positive direction (i think all axes became inversed). May I know if I miss soemthing ? Does it have to anything with Z=180 ?