Bar trade does not arrive evenly. A large share of an evening's sales can land within a couple of hours, and every staffing decision follows from that shape rather than from the total.
Demand arrives as a spike
People arrive in groups, and groups arrive around external schedules such as the end of the working day or the end of a performance.
The result is a curve with a pronounced peak rather than a plateau, and the peak is short relative to the opening hours.
The bar has to be able to serve that peak, because a queue during it produces walkouts that never appear in any sales figure.
Staffing sized for the peak is idle either side of it
A team large enough for the busiest hour is larger than the quiet hours need, and staff cannot be sent home and recalled within a shift.
Split shifts and staggered start times partially address this, at the cost of complexity and of asking staff to work an inconvenient pattern.
The idle cost either side of the peak is real and is one of the reasons drink prices bear little relation to ingredient cost.
The bottleneck moves as the night progresses
Early in an evening the constraint is usually table service and food, since people are seated and ordering slowly.
At peak the constraint moves to the bar itself, where the limit is drinks made per hour at each station.
Late on it moves again, often to the door and to security, which is a different skill set and a separate staffing line.
Preparation absorbs the peak in advance
Anything that can be done before service is done before service, because a minute at peak is worth several at any other time.
Batched drinks, pre-cut garnishes, prepared syrups and filled ice wells all shift work out of the constrained hour.
Bars that appear to be moving faster than seems possible are usually doing less work at the moment of service rather than working quicker.
Menu length is a staffing decision
Every additional drink on a menu adds preparation, stock and the chance that an order requires an unfamiliar build during the peak.
A short menu executed quickly generates more revenue in the constrained hour than a long one executed slowly.
This is why bars with serious volume run tight lists and hold their more elaborate drinks for the quieter parts of the evening, when the constraint has moved elsewhere.