It is extremely important to distinguish the difference between mushrooms and mycelium.
Mushrooms are the "fruit" and reproductive stage of these organisms, while the mycelium is the root system or "body" of the fungus.
Mushrooms contain much higher levels of active compounds like beta-glucans, and there is a large body of research on them. They have also been consumed for thousands of years as food, and many health benefits have been shown from just mushroom consumption.
Mycelium is commonly grown on a grain substrate, and this grain cannot be separated from the mycelium, so much of the final product is actually grain with no mushrooms present. This shows up in high amounts of starch and low amounts of active compounds like beta-glucans.
For these reasons, Real Mushrooms only utilizes mushrooms in all of its products, with verified levels of important active compounds like beta-glucans, and is always guarded against starchy fillers like grain.