Why were 7 miracles of Jesus called signs by John?

According to Strong's Greek lexicon, the Word translated "signs" in John 20:30 (semeion) means (1) that by which a person or a thing is distinguished from others and is known, (2) an unusual occurrence, transcending the common course of nature, (3) of miracles and wonders by which God authenticates the men sent by him, or by which men prove that the cause they are pleading is God's. Jesus' miracles were signs in the fullest sense.