Does this not imply that Mary knows full well that Jesus can and will do something rather extraordinary? Would that not further imply that she has experienced the more extraordinary side of Him before?
. He was announced by an angel
. He was conceived virginally
. His birth was accompanied by several supernatural events
. It was predicted that God had given him a very special mission
I believe that was enough to indicate that there was something extraordinary about him.
As for Cana, there is no evidence that she wa expecting a miracle. Obviously, Jesus had established himself as a person ready to help in all circumstances. She expected him to do it in this instance.