digiKam and Vignetting tool

The new Vignetting tool in digiKam is really cool but I have some issues with it.
The most irritating one is the fact that digiKam not only adds a dark oval around the image - but also lightens the centre of the image.
See below:
Unmodified - (PNG):

With Added Vignette (PNG):

See the nose?
It's much lighter. How much exactly?
I loaded both photos to the GIMP and measured the colour of the same pixel.

Pixel coordinates: X: 235 , Y: 351
ORIGINAL: 
Colour: Red: 197, Green: 191, Blue: 198

VIGNETTE ADDED IN:
Colour: Red: 234, Green: 227, Blue: 236