The output is still fully predictable by the artist.
The dice didn’t make the eyes, after all. They just showed 21, now it’s your job to actually make 21mm eyes. In doing so, you could mess up and/or intentionally make 22mm eyes. If someone asks, “Why are these eyes 21mm?”, you can answer “I decided to do what the dice asked”.
The dice are more like a client who asks you to draw a portrait with 21mm eyes. In other words, they are giving you a prompt. Nobody but you knows if they will get what they asked for.