Mushrooms are opportunists, so they grow where it is easy for them. Since there is little resistance on the sides of the grow kit, they often grow there more than in the middle of the grow kit.

The top covering layer is vermiculite and this is needed for these kind of grow kits as safety from continuations. It is recommended to leave this untouched, even though some people choose to remove some of the layer, so mushrooms have more space to grow. As said before, this is not recommended.