Why does steam in Chernobyl reactors speed up nuclear reaction?

The reactor type installed in Chernobyl (Wikipedia: RBMK) uses graphite as moderator. The water also moderates, but its effect is proportionally much less important than in a water-moderated reactor. This means a steam bubble forming results in only a small reduction in moderator efficiency but a large reduction in neutron absorption, thereby increasing the fission rate. This behaviour is referred to when the reactor is described as having a high positive void coefficient (the link goes to the subsection of the above Wikipedia article discussing this).