Why subtract PI*0.5 in NormalAngle() ?

[finscn]

In my opinion, NormalAngle should add PI*0.5 .
Why use - Math.PI * 0.5 ?

[finscn]

And I think the function NormalAngle and ReflectAngle are wrong (they are not real wrong).
the phaser's verion:

function NormalAngle(line: Line) {
    const angle = Angle(line) - Math.PI * 0.5
    return Wrap(angle, -Math.PI, Math.PI)
}

function ReflectAngle(lineA: Line, lineB: Line) {
    return 2 * NormalAngle(lineB) - Math.PI - Angle(lineA)
}

I try to fix some mistake , my version:

function myNormalAngle(line: Line) {
    return Angle(line) + Math.PI * 0.5
}

function myReflectAngle(lineA: Line, lineB: Line) {
    const angle = 2 * myNormalAngle(lineB) - Math.PI - Angle(lineA)
    return Wrap(angle, -Math.PI, Math.PI)
}

In fact the results of the twe versions are both right , sometimes they are the same , sometimes one is N , the other is N-360deg.

But in theory , the normal of Line/Angle shoud be clockwise rotation 90deg .

And there is an optimized without NormalAngle() :

function ReflectAngle(lineA: Line, lineB: Line) {
    const angle = 2 * Angle(lineB) - Angle(lineA)
    return Wrap(angle, -Math.PI, Math.PI)
}