Hi,
I can say one thing ,you are not understanding the concept of ATP process in SAP.Lets say,
Step1:
If you have always material quantity ,then if you keep any picking ,packing ,and transit times then system will go to backward process to consider all times to give requested delivery date (if you have always material quantity then system will give requested delivery as current date every time).
Step2:
If you don't keep, these times then system will give you requested delivery date as current date.
Thanks,
Naren