Hi Araving
The system behavior is correct. The schedule line delivery date should match the requirement date. That means, the material should be delivered on the requirement date, so that you don't need to keep it on stock for a long time.
The planned delivery time and purchasing processing time is used to calculate the opening date. Take a look on the picture below for more details:
If you want to have the delivery date before the requirement date, then you need to add a goods receipt processing time for your material.
BR
Caetano