I suspect that the first email is often not actionable because the cardmember fails to include all of the required information. This introduces delay while the concierge staff waits for clarification.
This would be my guess.
Working in communication, and see this all over the place. I always think about all the wasted email back and forth at various jobs trying to complete something, many involving 1 or 2 sentence messages that are sent that clearly require follow up because there is obvious information missing, and then even in the response, there is still info. missing (including things that I've specifically asked about). It's maddening. I'd be surprised to hear this isn't common with concierge requests.