Crap in the singular gives a, shall we say, scatalogical nomenclature to the game.
Dice with variable numbers of sides are not unknown on Roundworld. For an equal distribution of chance factors - i.e. an equal chance of rolling any of the spread of numbers - a prerequisite is that the dice be a regular polyhedron, and the familiar cube is but one of a number of these. While the consensus dice over the eras have been cubes, other variations used in Dungeons and Dragons and other board games include the tetrahedron - four sides; the decahedron - ten sides; the icosahedron - twenty sides; and of course the octahedron - 7a sides. It is quite possible that Discworld gamblers have learnt the potential of the octahedral dice, and use this for preference.