Vacuum Cleaners Discussions

I've come across this spec in Dyson reports and also for Central Vacs. However most other upright vac vendors call out motor power instead. Can anyone explain to me why there are two power ratings out there for vacuums? I understand what air watts are but I don't understand why there are two different specs. To use a car anaolgy is this like Horsepower vs Brake Horsepower?