The difference between the Sch. Start Date and Start date columns on project report, is that when the project is created a required field is the "Sch. Start Date" and it is auto-populated with the current date that the project is created.  Therefore, if it is July 21, 2016 and I create a new project in Project Insight 7/21/16 will auto-populate in the Sch. Start Date field for the project.  A project creator should put the estimated Start Date of the project but it is not needed.  Then once the project is populated with tasks a PM might set the first task of the project to start on a specific date by using a "Must Start On" constraint on the task and for this example I say they put the task to start on August 3, 2016.  Therefore, the result in this scenario is a Sch. Start Date of 7/21/16 but the Start Date will be earliest starting task which is 8/3/16.

Below is a video that demonstrates the above.
Online 7/21/2016