When a company/business is setting up their work schedule there are factors that they have to consider.
1. How many employees they need to accommodate with hours
2. If any one is on level or has vacation time in that time frame
3. Hours of operation vs. hours needed to be worked
4. Payroll account balance (they need to stay within a set budget)
5. Client or Customer traffic
These and more are considered when deciding on shifts for employees, they also have to figure in economic stability and that can change based on what time of the month and when people (customers) get paid.
Each business is different, so its hard to say how each schedules things but the basic factors that they go by are pretty much universal.