[drc]

I assume a card is usually graded Authentic because either it's altered (trimmed, whatever) or because the owner realizes it will be graded poor and prefers Authentic to Poor. The owner may think it looks better than the low grade it would get, so prefers the Authentic

Authentic can be assigned by the graders (almost always when the card has been altered), but the owner can also ask for that Grade.

With entombed cards that are autographed, it's a bit of a different deal. You can get a grade for the signature itself and for the card. Many autograph collectors are focused on the signature and don't feel a need for the card to be graded. They aren't into the PSA 8.5, PSA 7.32 thing. Many entombed autographed cards have no grade for either the signature or the card, just the authentic. Some cards have a grade for the signature but not the card. Either way, an Authentic grade for an autographed card doesn't mean the card has been altered. It often just means the collector wasn't interested in paying extra to get a numeric grade. All he cared about that the autograph was listed as authentic.