Mobility of a phase is its effective permeability divided by its viscosity. A ratio above one means the displacing fluid moves more easily than the oil, so it tends to finger ahead and leave oil behind. A ratio below one is the favorable case, a stable front.
In a waterflood the ratio depends on the relative permeabilities at the front and on the two viscosities. Thick oil makes it worse. Polymer added to the injection water raises the water's viscosity and brings the ratio down. Quoting a mobility ratio from absolute permeabilities, with no relative-permeability endpoints, understates how unfavorable a flood can be.