most likely an historical evolution
Here's how I bet it works (only Frank could tell us, and he's unlikely to comment on anything with Lu lurking in the wings):
When first designing the R22,Frank took some arbitrary power figure from Lycoming manual and designed/built components to match. This was 100% of derated power 'way back then.
As the design (pre-certification) evolved, it turned out that more power was (choose one) either a)needed, or b)acceptable in terms of life-limited components. So the usable derated power was boosted a few hp.
The engine RPM needed to boost the power this arbitrary amount was 104% of the original rpm at inital-concept-time.
Subsequently, there was no well-defined inflection point in the design-to-certification evolution when re-defining the new, higher RPM as "100%" would not cause confusion amongst the engineers and onlooking FAA types. God save us from Confusion! (Right, Lu?) So the new higher rpm has forever been known as 104%, damn the torpedoes, full speed ahead.
Just like CCW rotation and the right-seat-command-pilot and the prohibition on flying solo from left side regardless of ballast in the right, it's become a Tradition, and therefore Immutable.
(Is it true that the French are working on refitting imported R22's for CW rotation and left-seat command?)