In Cycles, the Subsurface Parameter of the Principled BSDF controlls the “Subsurface Scale” (Multiplier for the 3 Radius values). It “Mixes” between Diffuse and SSS not with an actual “Mix Shader” but with controlling the SSS Scale. This is as far as i know the right way to do it and more physically accurate than just mixing.
It can be seen in this tweet by Jonathan Lampel
In Eevee this parameter seems to be just that. A simple Mix between Diffuse and SSS rather that the “Subsurface Scale” - so that’s an inconsistency.
Also in my opinion,the Label should be renamed to “Subsurface Scale” to better represent what it does - or at least be better described in the Manual but that’s another story.
Edit: Did an update for the Manual - maybe it needs better wording. https://developer.blender.org/D3517
cheers