It's all about winding resistance, which correlates with core aperture.
For a center-tapped, two rectifier layout, only one half of the secondary is supplying current at any time, which is inefficient.
With the parallel secondary, bridge rectifier layout, the secondary is utilized fully. You'll have one extra diode voltage drop, which is only an issue at low secondary voltages.
My feeling is, that the 20% figure is a bit optimistic, my estimation would go towards 25%, but this is very dependent on transformer size.