Differences between Exception 20 and 15 in MD04

Hello experts,

Depending on the material, I have two scenarios:

Sometimes MRP says to reschedule the PO (message 10):

And other times MRP says to cancel a PO and create a new one (message 20):

Why does this happen? According to which data does occur one behavior or another?? Is there any value in the material master??

Thanks in advance.

Laura