Higher pressure causes the IP's internal plunger to advance. Under acceleration, the electric pump doesn't deliver that constant pressure and the timing retards a bit.
I'm set at 10* right now due to turning the IP down 1 flat after setting it at 9* and the issue I have with that is hitting higher EGT's quicker. I think 9* may be the sweet spot on my N/A IDI with the Duralift. The 4 psi mine makes at WOT is still more than what the mechanical lift pump makes however. Sometimes it'll only run like 5-6 psi at startup and cruise. When I turn off the kill switch for the pump while running, I can hear the timing difference (less clatter) and doing it with the pedal down I can feel the difference that little bit of fuel pressure advance gives.
Oh, and I have my pump powered via a relay that is triggered by the FSS power and a 3 prong oil pressure switch. One terminal gets input, one output until oil pressure builds, then it switches to the other terminal for output which would cut all power if the engine losses oil pressure so the pump doesn't continue to run (all outlined in the link in my sig as well as the priming circuit).
If you're still running the Duralift, I wonder if it could have anything to do with the porting from 1/8" NPT to 1/4" NPT that you did?