How are shipping methods/transit times defaulted during Sales Order Scheduling?

Summary

How are shipping methods/transit times defaulted during Sales Order Scheduling?

Content

How are shipping methods/transit times defaulted during Sales Order Scheduling?

My Transit Times setup between Warehouse and Customer Location is as follows - 

Shipping Method A with 3 days
Shipping Method B with 2 days

I notice that in all cases, regardless of the Request Type (Arrive On/Ship On) or Onhand availability, it is Shipping Method A that gets defaulted with it's 3 days transit times.
This leads to inaccurate scheduling results, especially in cases where onhand is available and shipment (ship on/arrive on) is expected to be on time.