Don’t know whether this was the actual reason, but one advantage in doing so was that it clarified that it was to count as a new name. It is not unknown for monarchs using two names to apply the regnal number only to the first of them. So the current King of Sweden is Carl XVI Gustaf. Explicitly using the name John Paul I ruled out the possibility that he was John XXIV Paul.
“Who?”
“Cardinal Hu? He’s from Beijing.”
“Who?”
“Yes, Hu”
“What?”
“The Pope?”
“Who is the Pope?”
“No, Hu is the cardinal from Beijing”
“I don’t know! I’m asking you!”
“Patrick Yu, the Anglican Bishop of Toronto? What does he have to do with this?”
I personally know two brothers called Hitler and Mussolini (parents were Lebanese Catholics), the priest refused to baptize them with those names (they are in their 60s), so they have Christian names and legal names.
FTR, names like Stalin, Lenin are also common in Latin America with people whose parents are… er, not well-read.
Yesterday, Francis, occupying the chair of Peter, responded to Bartholomew, Patriarch of Constantinople, who occupies the chair of Andrew, and said to him, “Thank you my Brother Andrew.”