Basically, the figures are a measure of kinetic energy, not a measure of stopping power. Kinetic energy varies with the square of the velocity and hence fast moving projectiles have a high kinetic energy figure. Living tissue is good at absorbing energy without too much effect. At typical handgun velocities, energy levels haven't crossed the threshold where it overcomes the energy absorption levels of living tissue (this may not apply to some of the rifle type cartridges fired from some of the specialty handguns like TC etc). Stopping power has a whole lot of factors (plus random chance and dumb luck) Will the projectile penetrate far enough to reach vital organs? Penetration is largly influenced by momentum, sectional density and projectile construction. If the projectile reaches the vitals, will it cause enough damage? Will it leave a large enough wound channel to cause a rapid loss of blood pressure? These issues are often related to diameter.

Don't worry, even Governments with massive budgets and immense research capacity frequently fail to understand the parameters. Plenty of police departments and military make the mistake.