Thank you for a fascinating insight into your design process.
Brian Lee Yung Rowe

Thanks for you reply Brian. The Glue team would love to take on the process of independently and objectively testing out every single variant of a component, and the different combinations of these components. But we are still a team working for an existing product and brand.
So when we choose which variants of a component we’d like to test, we are influenced by our design principles and our brand. That is also part of our responsibility: we look at how the brand is represented in our product and how an experience can feel unique and key to Spotify. 
Making sure that influence doesn’t become a bias, comes down to finding that balance between the art and science of creating a good design language for a digital product. Our tests continuously challenge and validate the design principles and brand values.
The question remains on what basis elBulli chooses their ingredients they want to experiment with during those 6 months. I can only assume it’s impossible for them too to test every single ingredient.