this is from wiki:

a statement also made by Diana's royal protection officer, Inspector Ken Wharfe. Wharfe reiterated this in an interview in 2016,[26] and in his memoir Diana: Closely Guarded Secret published in 2002 (and reissued in 2017) in which he says "A simple comparison of dates proves it is impossible for Hewitt to be Harry's father. ... Harry was born on September 15, 1984, which means he was conceived around Christmas 1983, when his brother, William, was 18 months old. Diana did not meet James Hewitt until the summer of 1986."

Diana attributed Harry's red hair colouration to her side of the family, calling him her "little Spencer".

It would make sense for a reason the king has to remind everyone that Harry is not a working royal though lol