Like the fact that people like to decorate stuff and simple geometric patterns are an obvious way to do this (like the dog-tooth design around a Norman arch, or a zigzag design around the rim of an urn). Like the possibility that the motifs had particular meanings, now lost, that related to the culture/religion/landscape/favourite football team/whatever. Like the fact that designs often relate to the shape of the block being carved. None of which has to necessarily have anything to do with mushrooms.