It's similar to shopping for a car. Simply choosing a vehicle with the largest engine won't ensure the best performance or the fastest speed. The car's weight, tires, design, and purpose all come into play. The same holds true with electronic flash units. The basic concept behind electronic flash is relatively simple.
Rechargeable batteries hold energy and, when triggered, release stored energy to the flash tube.
Although not really rechargeable batteries, they function in much the same way. The primary difference is that rechargeable batteries charge and discharge over a long period of time, while flash unit capacitors charge and discharge over considerably shorter time frames. Just as engine size will not solely determine a car's efficiency, the amount of stored energy will not be the only factor determining a flash's performance. Choosing equipment based solely on watt seconds will not give a true representation of the equipment's capability. From the system design to the quality of components, every aspect determines how much light is actually produced.