I feel like I have answered it; an "IRL value gained from an off-wiki source" is a constant or a scientific measurement; it is something fundamental that we used which we don't treat as being actually calced. The speed of light, the speed of lightning, the melting point of steel, etc. Sure, these are things that can be "calced" but they're objective for our purposes. Even if they're arguably "calc stacking" because you can't calculate anything without making basic measurements first, we do make allowances for calc stacking in some scenarios and this would be one of them, otherwise we'd never calculate anything at all.
Where a "value gained from a wiki calc" is based on your interpretation of what is happening in a feat or a statement. There isn't an objective measurement of the "power to obliterate a forest"; you have to assume the size of the forest, the method of destruction, etc. You introduce variability because of these assumptions and therefore the result of the calc isn't completely fixed.
So it's not just a matter of it being on-wiki vs. off-wiki; it's about comparing something that is constant and fundamental to something that is based on assumptions and interpretation.