Scaling, mirroring, how does it really work? I don't really understand this behavior :

All is done in object mode.
Here are 6 identical shapes, from top to bottom rotated around the Y axis, respectively 0, -45, -90, -135, -180 and -225°.

Now I scale them -1 with 3D cursor as pivot and along the X axis (shapes on the right are 'witness') :



Now, the scaled shapes, from top to bottom (all kept their original rotation around Y) :


1 : scale X = -1, effectively mirrored (like from a mirror)
2 : scale X = -1, but not mirrored ?
3 : scale Z = -1, and appears mirrored
4 : scales X and Z = -1, not mirrored ?
5 : scale X = -1, mirrored
6 : scale X = -1, not mirrored ?


Why all that ?

Note : I don't want to obtain any particular result here (I know how to correct things if needed), but I just want to understand why it works like that.
This matter because you don't apply rotation to original objects.
"CTRL+A - rotation" should solve this.