Usually when the credits roll for a movie you see the director’s name with the words “directed by” and the writer’s name with the words “written by” and maybe some producers and executive producers and co-producers, all with those titles.
Then come the actors’ names without explanatory ornamentation of any type. Could be six names, could be twenty. Usually you know who the first two names because they were the stars, then you’re left with a puzzling few minutes…Bethany Eastwood, was she the best friend? Do you think she’s Clint Eastwood’s daughter? Patrick Bridgewater…was he one of the guys in truck? Wait, that name sounds familiar, I think he’s on that TV show, you know the one with the cops, the dark haired one.
Then some more credits like costumer designer and composer, THEN a full cast list that says who played who.
WHY do they do that? Why not skip the first listing of the actors names with no accompanying information?
I wonder if it’s a branding thing. It’s kind of like how you never see a commercial for Coke that explains how it tastes, and how good it is. Do they really need to convince us that it’s a delicious beverage? No! We already know. But a commercial that shows cute polar bears and a logo? That reminds us to pick up a case of yummy beverage.
It may be the same thing here. We don’t need to know that Danny Ocean was played by George Clooney or that Ellie Arroway was played by Jodie Foster.
What you describe is often the case is a film with a cold open or limited opening credits. For example a Star Wars movie that goes straight into the, " In a galaxy…" scroll. Since the stars weren’t billed as such in the opening they do it during the closing credits before the full cast list.