I have been researching this ALOT this week as I thought the same but I have come to the conclusion I actually need to up the EC, something about the plants roots having it's own internal EC so if your feed is lower then this the roots actually leach where they are trying to match the EC of the res or something like that lol!
So I have upped the EC now which I'm hoping will do the trick if I'm right, fingers crossed as it's a bit of a ball ache to keep adjusting PH everyday, gone through more PH up during this grow then I have in 3 years of growing!
Crazy, makes sense. Jorge Cervantes book says it's better to flush with a low nute solution rather then pure water. Probably has something to do with ^^that^^