I'll try to explain :
The buffer is wide enough to allow you to DC a dp move in to a new dp move by simply holding df and press the button for the DC after the first dp.
By example you can do dp.C,(DC)dp.AC with Kula by doing : 6523+C~AC (Because moves can be drive cancelled by their Ex versions). But you can also DC the dp of yuri in her demon flip : 6523+C~D, same thing goes for qcf moves, you can DC Kensou's rekkas by doing qcf.A,qcf.A~AC (In fact you can also cancel the second rekka by the third with the same idea).
What's why sometimes a Ex dp comes out, you're not yet on the 6 you press AC the Ex dp comes out.
Now to avoid problems it seems snk makes the game recognize buffer like 6523+C 6+C and prevent it to be drive cancelled. But this thing is disable in Maxmode what's why this trick will works really well. And doing dp.C,(HD)qcf.C by this method is really easy. So if you're not using this in Maxmode i can only recommand you to do the real move, 6523+C 236+AC. It will avoid any problems if you're precise enough.
/* About doing the dp.A,(DC)qcf.AC with K' */
Since i'm not a K' player it tooks me a while to understood it's a timing problem.
Btw i asked the french community and the answers are :
You can do 6523+A 6+C/AC but the window is really, really tight (i thought it was impossible my bad)
You can also do hcb f.A,(DC)qcf.AC the window is really tighter than the A or C version but i succeed in some times.
You can also do 6236A, 9AC but here the timing is tight too ...
I don't know for this specific case which one is the easiest.
If it's just for the trial go into HD mode.