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This question has got to be the reason that Profoto doesn't mention that number much anymore.
It has an internal reflector because it has an internal flashtube. The designation of that reflector is immaterial; what matters is the beam angle of the light the whole unit produces, and that's a whole heck of a lot wider than 77°. For a point of reference, a Zoom Reflector set to its widest—a 105° beam angle—will visibly and significantly reduce the beam angle of the D1. (It will also increase the efficiency.) The reflector's there to serve the same purpose as the reflectors on Elinchrom products and the Broncolor Picolite: to push the light forward instead of wasting it.
Think of putting a flashtube at the bottom of a soup can. When you pop one off, most of your light's going to the sides and back, not out the front. You can solve that one of two ways. You can either add a reflector at a cost of effectively nothing, or you can add more capacitors and a beefier flashtube, at a cost of price, heat, power consumption, size, weight, and speed. And in the D1's case it's more like a tuna can. It's recessed just enough not to get smashed, so there's very little to obstruct the light.
Where the stock D1 may not be ideal in some situations is with modifiers like beauty dishes, and even that's largely a personal preference issue. If you slap on a beauty dish and don't like the way it looks, you can swap the frosted plate for a frosted dome and end up with a head with output as similar to any other Profoto head as any other Profoto head is.
So where's the 77° come from? Heck if I know. Even run totally bare with no plate, the output's much wider than that. The best I can give you is that the reflector surface itself is physically around 80 degrees, so maybe they're basing it on that instead of actual light output.