You're noticing a problem that unfortunately is a characteristic of cable-operated rear brakes. Any cable will stretch a certain amount under tension. Let's say the front cable stretches X amount under tension; the rear cable is so long (compared to the front) that the amount it stretches is something like X times 3 or 4.

About all you can do is buy the best cable you can get for the rear, make sure the wheel is dead-on true, so you can adjust the pads as close as possible, and use good pads. Sometimes those old rod-operated brakes don't seem like such an obsolete idea.