You *ARE* seeing a top-down picture. Take a triangle, and extrude it thru your playdough machine until it has depth. It's three rectangles on the sides, with two triangles for endcaps.
Might be easier to visualize if you take a carrot, cut off the ends, and whittle it down until it's triangular in profile. Then what they did was pogressively cut it 'shorter' until the balance was right and all sides came up equally often.
It's just tough to visualize when we're all used to dice with equally shaped/sized faces.
edit: Now I want to go home and make myself a set of carrot-dice. Fun to use, and you can combine them with onions and celery to make a nice mirepois sauce-base in a pinch!