DOB Conundrum

My ancestor James McMillan gives his DOB on the 1939 register & other sources, (R Navy, he was at the Battle of Jutland) as 24/12/1889 in Plymouth/Keyham/Morice Town and also states, on the 1939 register that he is married. I have had no success in tracking down a marriage or his birth registration. However, as a last resort I ordered a copy of his birth registration from Plymouth Registry Office and they supplied me with a certificate for James, showing his DOB as 24/12/1892. My query is, why did he amend his age to make himself 3 years younger? Wouldn’t he have had to produce his birth certificate when he joined the Navy? He was in the Navy from 1908 to 1926. I know young men lied about their age to join up, but that was usually to make themselves older, not younger surely?
Thanks for any suggestions!