I cannot reproduce this.
With 160 passengers, you will see a large bus carrying the first 90 ( max capacity for the large bus ) and another large bus with the other 70.
With 184 passengers, you will see exactly 3 large bus.
Please provide the precise, exact, reproductions steps, like the exact order you choose the services and when you loaded the Simbrief plan, and the exact number of passengers when this happened.