People often get "biodegradable" and "compostable" mixed up but they do not mean the same thing. A biodegradable product may be broken down by microorganisms but this does not necessarily imply that the product can be converted into good quality compost.
Biodegradability and compostability rely heavily on the environment where the product is broken down. As each environment (compost, soil, water, …) has different temperatures and microorganisms, the speed of the biodegradation process may vary from one site to another.
For example, bioplastics which are biodegradable in an industrial composting plant (the most aggressive atmosphere regime) are not always biodegradable in water or soil, or even in a compost bin in the garden (owing to the lower temperatures).