Measurements of the volume and surface resistivities of electrical insulating materials
They don't attempt to explain it in the paper, as you know. I would think (aka guess) it has something to do with the interface of the electrode and the solid. A vacuum-deposited silver electrode is in intimate atomic-level contact with all the surface details of the material, whereas a conductive rubber is probably only touching at microscopic high points, despite the apparent compliance of the rubber- so the apparent conductivity appears to be lower (higher resistance). 

As to whether reading high by 50-300% is a big deal, my reference data for bulk resistivity of insulators tends to give numbers within pretty wide range- for example polycarbonate 
has a stated resistivity in the range of  \$10^{12} \$~\$ 10^{14}\Omega\text{-m}\$, which is 100:1 not 3:1.