Obviously you have never been in Kaohsiung at 10:00 pm when a 30 second delay in the call for push made you 30 minutes late. We used to call for push as the last passenger was walking up the air stair and would be ready within 10 seconds of that event. If we didn't do that then China Airlines, Eva, Formosa, Makung, Trans Asia, and Uni Air would all be ahead of us.
The moral of that story is that some crews are conditioned to call for push back anticipating being ready in very short order. Sometimes they are and sometimes they aren't.
Typhoonpilot