Charisma really is a very strange trait. It is a force of personality, but isn't personality, well, personal? And therefore not racial?
(This is why my interpretation of tiefling charisma and intelligence is that their fiendish blood literally makes them magically more persuasive and sharp-minded)
Yes, I think that is the only possible explanation.
Charisma, in our world at least, is the only ability that is relative: you always need another living being to make use of it. But the world of D&D seems to be different: sorcerers can use their charisma to create force fields or set things on fire. No other living creatures are required to project their intimidating personality on. So in D&D, charisma seems to be some form of magic. Or a power source for magic.
