That doesn't make sense - the word statutory means "enacted by statute" which is exactly what the Sexual Offences Act does - it is because a piece of paper, (Sexual Offences Act), says that someone below the age of 16 cannot give consent that the offence is called "statutory rape".
Both would be instances of rape but as is the case with all offences there are different degrees of seriousness. Whether a child steals a packet of Polos from the corner shop or millions are stolen from a bank or a pensioner has their life savings stolen the crime is still theft.
That is exactly what statutory rape is - sex without consent - the Sexual Offences Act 2003 states that a person under the age of 16 cannot give consent.