I've noticed different mediums produce different readings and unless you're using the method Canna recommends to test for ph I doubt we're that accurate with most methods used. I once went through hell over analyzing runoff and in the end it really didn't matter. I've flushed plants with 8.0 water and had runoff measure 5.8. I've flushed plants with 7.0 and 6.0 also with no big diff. in runoff ph. Once in a while I'll measure the EC of the runoff but only if I see symptoms worth investigating.