There is an important difference between IBIS waveform initial delay and the initial delay delta calculated by the DDRx wizard. Removing initial delay from the IBIS waveforms will generally not affect the timing calculations of the DDRx wizard, as you found. Initial Delay Delta is calculated from the timing models as explained in detail in this application note,
Let me know if this application note does not answer your question.
thanks for the feedback. It's clear now that the initial delay isn't the same as the initial delay delta.