The children of George V were born when different rules applied. The current situation regarding royal titles is governed by the Letters Patent of 1917 which provide that the title of prince or princess is limited down the line as far as the grandchildren of the sovereign in the male line, except that the eldest son of the eldest son of the Prince of Wales has the title of prince. The Queen extended that to all of the children of the eldest son of the eldest son of the Prince of Wales.
So if these rules had applied to Queen Victoria's grandchildren and great-grandchildren, the Queen's uncle who became Edward VIII would have had the title Prince, but not the others while Queen Victoria was alive.