Difference between PFI Port and DI

Hello everyone
I have a DAQ card: PXIe 6378, with which I want to implement a digital trigger.
If I use the PFI port, the trigger triggers completely synchronously if you make the same attempt more often. If I use the DI as a trigger signal, the trigger triggers sometimes earlier and sometimes later. What could be the reason for this? For information: I have set a pretrigger in both cases.