vector.js same phi for two different vectors

[luckylooke]

Copied from console:
a=new Vector(1,-1)
Vector {x: 1, y: -1, z: 0, negative: function, add: function…}
a.toAngles().phi
-0.7853981633974482
b=new Vector(-1,-1)
Vector {x: -1, y: -1, z: 0, negative: function, add: function…}
b.toAngles().phi
-0.7853981633974482

Equal case is for a=new Vector(-1,1) and a=new Vector(1,1)

[bowbahdoe]

Can you show mathematically that this should not be the case?