I am well aware.
Ironically, the only time this doesn't happen is when the the aperture is set to 2.8 at 18mm and you then zoom in. Indeed the aperture does change to f4 but with no obvious change in exposure.
As a side note, this is also happening when taking stills. I have taken a set of photos with fully manual settings and they all vary in exposure despite all the data (as seem in Lightroom) being exactly the same. Not good.