Because the man was Ernest Hemingway and the bar was the Floridita in Havanna, Cuba.
Rum expert Matt Robold makes a very convincing argument that a Hemingway Daiquiri and a Papa Doble is not the same in this essay.
Another great essay about the origins of the Daiquiri and the Hemingway special is found here.
I just figured, that since I had to kill a whole, lovely grapefruit for my drink, using a teaspoon full of the juice was a waste:
- 6 cl white rum
- 2 cl grapefruit juice
- 1 tsp lime juice
- 1 tsp gum syrup
- 1 tsp Maraschino liqueur