How is the age of the Prophet Muhammad, peace and blessings be upon him, calculated as 63 years, whereas his birth in 570 CE and his death in 632 CE total 61 years?

The Prophet Muhammad, peace and blessings be upon him, was born on Monday, the ninth of Rabi' al-Awwal, in the first year of the Elephant incident, corresponding to April 20th or 22nd, 571 AD. He passed away, peace and blessings be upon him, at the age of sixty-three lunar years, which is equivalent to sixty-one solar years. His death occurred in the forenoon of Monday, Rabi' al-Awwal 13, 11 AH, corresponding to June 8th, 633 AD.