If your legs only drill results in a shorter distance than your upper body drill, I contend that you weren't pushing hard enough with your legs.
Altough this may be true, the upper body makes a longer contribution to the total pull distance in each stroke. Try a 5 minute row with only the upper body, and then a 5 minute row with only the lower body, each one at the same SPM, and compare the distance rowed.
Even if your hands moved further during the "pull through" than your butt does during the leg drive portion of a stroke, (which it shouldn't), the force your legs generate is much bigger than your upper body generates. The distance ticked of on the PM is based on the work input. Work = force*distance, and I'd say that the force my leg drive generates at least 50% bigger than the force I generate with my upper body, my leg drive would have to be 2/3 or less the length of my upper body pull through for the legs to be less significant to the distance. Leg drive definitely isn't that much shorter, and I contend that it's probably longer unless you are built like a gorilla and have really long arms proportionately.