@pastanaga - It's unknown because it's fairly complicated. Hair growth might not seem like it's all that mysterious, but the hair goes through quite a few stages of growth and they aren't all that straightforward.
Humans like to think they have the natural world all figured out but there are quite a few, if not most medications that we use because they work, not because we really understand how they work. Steroids are no different.
If we had a true and deep understanding about how everything worked, we wouldn't need to test medicines so much. We'd just be able to put together chemicals and know exactly what they'd do.