At one point, I worked for a large (50,000 person +) Canadian multinational which had a mix of union and non-union business units and divisions.

The way it was explained to me was that variable comp like this was only offered to staff not covered by a negotiated agreement (i.e. non-union shop). The intent was to dissuade further inroads by the various unions by using this as both a carrot and a stick. This strategy seemed to work, at least at my location. I make no claims that this is AC's rationale however.