Because of how the transmission is made
When you empty the ATF only 2.5 litres comes out
As the other 2/3 are held in the wet clutch packs of the transmission

So if you want to do a 'full' change you'll need about 9 litres of ATF DW-1 which will set you back at about $160
So what you do is
Change ATF - drain, fill up
Drive around
Engage ALL GEARS - find a quite place and drive it up all through the gears (60 km/h should get you engaging the torque converter)
Then let the engine declerate to 0km/h congratz you've changed your ATF
Now repeat this process 2 more times and feel the smoothness of your AUTO for only a while longer... then back to crunching