* George, Charlotte and Louis were made princes and princess by letters patent to get around the "grandchild of monarch" rule. Otherwise they'd have to wait until Charles acceded the throne.
For some reason, Archie was not going to be made a prince at birth in the same way - he would have to wait until Charles acceded and it was automatic by right of law.*
The first Black prince of the Blood of the UK.
That's just deliberately shit isn't it.
It wasn't to get around the 'grandchild of the monarch rule' though. It was to avoid a situation of W+K having a firstborn girl (before they knew George was male) who wouldn't be entitled to be 'Princess' but would be third in line to the throne due to the Succession Act replacing male primogeniture with absolute primogeniture, because the first boy they had (even if after their first child had been a girl) would be HRH Prince RoyalBaby as a result of the Letters Patent from 1917 entitling 'the eldest son of the eldest son of the Prince of Wales' to be styled as such.
It was to correct a possibly weird situation.. If Charlotte was the eldest she'd have outranked her brothers in terms of line of succession but been outranked by one of them in title, which makes no sense at all and causes difficulty in terms of rank in the family. None of that is relevant in Archie's case, so the normal law applies.