Time varying arrival rates are random. Excerpt from Simio Help:
The Rate Tables panel in the Data window allows the user to model situations where an arrival/event rate varies discretely over time. The number of fixed rate periods and the length of each rate period can be specified in the Rate table. The rate pattern automatically repeats during the running of the simulation.
A Rate table is used by the Source object / Timer element to generate entities / events with a time-varying rate. Internally, a non-stationary Exponential distribution is used to calculate the rates. The rate units for the Poisson arrival process is Arrivals per Hour, regardless of the time units specified for the intervals of the rate table.
In order to use a Rate Table with the standard Source object, set the Arrival Mode property in the Source to 'Time Varying Arrival Rate' and then select the appropriate Rate Table for the Rate Table property. The Source also includes a Rate Scale Factor property that can be used to easily modify the values within the table by a given factor instead of changing the values separately. For example, to increase the Rate Table values by 50%, simply specify the Rate Scale Factor within the Source to '1.5'.