"To increase the variability" means to make something more diverse or different. It refers to the action of introducing more variety or variation into a particular situation, system, or group. Full definition
However, they did find increased variability in the surface contours among some of the later styles of points, indicating that those tools had not been produced using a consistent technique. (sciencedaily.com)
The entry of more women into the workforce also led to improvements in kitchen appliances and increased the variability of ready - to - eat or frozen foods available in the market. (fao.org)
The point about increased variability is a rather minor one I would have thought. (realclimate.org)