Flush draw odds

How come the odds of hitting your flush card on the river are 19%, but the odds for the same draw with both the turn and river remaining are only 30%? I thought it basically doubled, as one of the quick ways to determine approximate odds is to multiply your outs by 4 and then add 2 after the flop and multiply by 2 and then add 1 after the turn. Most of the time this seems to work out almost exactly to what the %'s are shown to be on television, but the flush draw odds don't seem to follow the same pattern. Please explain.