Some say that, while the game runs internally at 224x256 (8:7), it's "intended" to be displayed at 4:3, but I don't know about that.
The H-SIZE and V-SIZE pots on the DK monitor are free-adjusting, so it's really whatever aspect ratio the user wants.
All I know is that if you adjust a real DK to fill the horizontal space, it always ends up looking much closer to the MAME 8:7 than the default 4:3.
This is actually a significant issue. Whatever you've gotten used to really affects gameplay if you have to switch, especially on springs. Even a minor aspect ratio difference can totally throw things off and take hours to adjust to.
In any case, I don't know where anybody's getting the idea that 9:6 is correct.