SA Schedule Lines from MD03 date calculation

Hi

Please find the following snap from MD03 ,

here the schedule lines generated dt 06-06-14, for this material planned delivery time and GR processing time is zero in material master,

In info record planned delivery time is 1 day, I unable to understand why system generated the schedule after 4 days

Then I created new material copying the above material and done MD03 for newly created then the schedule lines generated on 02-06-14, Why the system proposes two different schedule dates for new material and ol material?

I searched the forum and checked all the available configurations and finally found

IMG-Production-MRP-Planning-scheduling and capacity parameters-Parameters for determining the basic date

when i checked the box for Start in past and again run the MD03 for the same material, then the results as follows, which is correct i think as per MMR

Though i got the required results but did not understand what is this field Start in Past does? how the calculations takes place, what will be the effect if i keep this checked ?

Thanks