Why is there a reference pressure for EqLA results?
(Or Flow, Permeability, ACH results?)

The results are all referenced to a single pressure, the reference pressure.  The reference pressure is dictated by the standard with which the test is trying to comply.

Induced building pressure is directly proportional to airflow rate through a building, and EqLA is directly proportional to airflow rate; so that a higher induced building pressure would be a higher EqLA. Hence, there needs to be a reference pressure when referring to a leakage or flow result.

For the ASTM standard, the leakage result is required to be at a reference pressure of 50 Pa, whereas it is 75 Pa for the USACE standard. Reference pressure also applies to permeability (e.g. CFM/ft2) and air change (ACH) results.