1. Passenger asks for specific drink.
2. Crew is confused.
3. Crew asks colleague.
4. Colleague fetches the ingredient.
5. Passenger served.
It's called training. My guess is that crew do have a list of everything, are trained, but may not have 100% infallible memory. Thus, they ask a more senior crew member who makes it right. Perhaps with more poise, a more senior crew member would have thanked the passenger and then quietly asked another crew member for the information. But, the difference is hardly significant.
This isn't even a first world problem. It's not a problem.