[tooseevee]

The real horsepower of the 1922 RE was probably known to the engineers in 1922 but the published horsepower of 2.25 was based just on the engine displacement for tax purposes. 
Note that the 2.25 horsepower is 10 percent of the 225cc displacement.

That makes as much sense as the way they calculated horsepower in Europe and the UK for the cars at that time.

They used a "RAC" rating which was based on the bore size and the number of cylinders.  Stroke had nothing to do with it so this system drove the designers to use really long stroke designs.
The RAC horsepower was equal to the bore (in inches) squared multiplied by the number of cylinders  divided by 2.5.  (Bore^2 X n)/2.5.  = RAC horsepower

This gave the supercharged 8 liter Bentley 6 a RAC horsepower of 45 while the OHC 4 valve/cyl. engine was actually cranking out 220 brake horsepower.

Our 500cc single using the 10 percent rule would have a horsepower of 5.0.

       ....and that formula gave the 2CV its nickname; Deux CheVaux-vapeur (steam horses), Two Tax HP. Many other cars of that era got their pet names from that formula for taxing vehicles.