Not sure of an official difference, but my take is a grift is something that everyone's kind of doing on the DL, but nobody is admitting that it's a scam.

Think like a cult. Everyone's a part of the cult, but nobody actually wants to believe they're getting scammed or scamming others, so it's more of a grift. People assume what they're doing can't last/sustain, but they do it anyway because the benefits are good.

A scam is straight up the party knowing it's illegitimate and going out of their way to execute the scam so they can benefit at the expense of others.

Basically, I've always taken it as one is self aware (scam) and one is only self-aware at the top levels (grift).

But this is all just in my head.