Where the scope lives
With an API key, the scope is a property of a record in the exchange's database. It is respected as long as the exchange is honest and its systems intact. With an agent, the scope is a property of what the signature can express: there is no withdraw action available to an agent, so there is nothing to misconfigure.
The difference matters most in the scenarios you cannot audit — a compromised operator, an insider, a support tool with too much reach.