I'm confused, there is no approximation ?

[bot66]

In the README.md introduction:

Finally, we compute the AP as the area under this curve (shown in light blue) by numerical integration.
No approximation is involved since the curve is piecewise constant.

But I look into the code (line 185 main.py)

    """
     The Average Precision (AP) is the area under the curve
        (numerical integration)
        matlab: ap=sum((mrec(i)-mrec(i-1)).*mpre(i));
    """
    ap = 0.0
    for i in i_list:
        ap += ((mrec[i]-mrec[i-1])*mpre[i])
    return ap, mrec, mpre

The calculated AP, is the area under the red line not the light bule line, isn't it ?
But README.md says that is the area under the light blue line.