[betty boop]

Serving to explain; demonstrative.

Brother Arlen, they both "demonstrate." The issue is by what method of reason.

We need to get a hold of cornelis right about now. If anybody can "referee" our little dispute, it would be he.

[unspun]

It may disappoint, but if had to bet, I'd say you were accurate and the dictionary.com definitions are the results of "the telephone game" that has been going on with the terms over the last 2.X thousand years.

Now, maybe we should move on to the most primary definitions of "incredible" and "subjective."