Resource Assignments / Crew Scheduling

Program P48330 Resource assignments

When a work order has a routing Start Date later to the first current period day,

why when assigning a resource to the routing the system assigns the work order order to first current period day and not to order start date?

For example

Today is 20th and I have  an order with start date 20th  and the current period is from 16th to 20th . When I assign the a resource to that work order, the system assign the resource to december 16th.

Regards