Randomized blockstates do not apply full models

When a blockstate file defines several block models that are intended to be randomly chosen, rather than picking one full model per block the game instead somehow selects each individual face of the cube separately and combines them into a new, full cube.

I have included an example blockstate file that changes obsidian to randomly display as either stone, cobblestone or stone bricks. As you can see the obsidian blocks are not rendered as cubes that are completely stone, cobblestone or stone bricks. Instead the six sides of the model get randomly assigned to one of the three textures.