Relief is everything. The string is a straight line between the bridge and the other contact point, the nut or where your finger pushes it onto the board. If you've a lot of relief, it will exaggerate the action at the lower end and, if you set the action low, you're effectively guaranteeing the notes will buzz higher up the neck as the actual string height (when you've effectively ruled out the extra clearance provided by the relief) will be too low there. If you want to check this for yourself, hold a string down at the 15th fret and look at the string clearance to the end of the neck. Now do the same around the 3rd fret. See?