Gilles
You raise an interesting point. George VI was Admiral of the Fleet as was his father George V.
The first set of George V definitive stamps feature the King wearing his Admiral of the Fleet uniform, hence the nickname for the series, "Admirals".


When Canadian stamp collectors refer to the "Admirals" this is the issue they are referring to.
In 1942,
Canada Post Office issued a set of definitive stamps featuring George VI wearing the uniforms of the three services.
1. Navy


2. Army


3. Air Force


It is obvious why this is called the "War Issue"
This is George VI wearing his "Admiral of the Fleet" uniform:

Not the uniform used for the War Issue stamps.
For more on the War Issue:
http://postalhistorycorner.blogspot...42-1943.html