tl;dr it's just what that line often uses as a last name when pressed.

it's a possible 20 year violation. It's not actually a practice/official policy. Instead William (who went to school under the name william wales) and Harry sometimes use the term Wales as their last name given their father's long term association with the role.

They arguably should be using the name Windsor-Mountbatton but for whatever reason they choose not to (since even though harry was born in 84 and thus may technically fall outside of 20 years i'm keeping this response light on deep semi recent history dives)

unlike /u/robotnixonshead 's excellent example this just seems like a really boring answer to a potentially interesting question.

This post has been approved for now, but after a little light digging I have the sneaking feeling this is a fairly recent practice, possibly falling afoul of this subreddit's "20-year rule".

Potential respondents: do keep discussion to events/conditions prior to 1995 only.

OP, you might consider x-posting to a regional general (e.g. /r/AskUK) or military sub.

When the future George V was in the Navy, with his elder brother, they both used the surname Wales, as their father, the future Edward VII was the Prince of Wales. It's a well established convention.