From the Magnification Wikipedia page, I have the following equality:

M = d_i/d_o = f/(d_o – f) = (d_i – f)/f

with M the magnification, f the focal length, d_o the distance from the subject to the lens, and d_i the distance from the lens to the sensor.

So, when the magnification is 1 we should have d_i = d_o = 2f.

With my macro lens (EF 100mm f/2.8 L IS USM Macro), the minimum working distance (from subject to sensor) is 30cm, at this distance the magnification is 1. From what I unsderstand from the formula, this distance should be d_i + d_o = 4f = 40cm.

So I think I missing something, can someone explain where I'm wrong?