It is just based on the unique histories of those areas and different understandings of those terms. Wales, Scotland, Northern Ireland, England and the rest of the UK were historically separate countries for many years. The did not join together in the UK at the same time, and at one point, the Republic of Ireland was part of the UK but left in 1922 (after being a part of it for more than 100 years). Scotland of course has recently debated and voted on whether to leave the UK (deciding against it). Although they are still considered countries for many purposes, they also are subject to the government of Great Britain and some of the powers usually held by a country are actually held by the central government, not the governments of those separate countries. The United States initially looked like 13 countries with a loose federation ("state" can sometimes be used to mean a "country"), under the articles of confederation adopted shortly after the revolutionary war. However, the United States later adopted a constitution that gave the federal government stronger authority and took a fair amount of authority from the states that are usually associated with a country. Over time, the federal government grew in size and significance, with state governments losing much of the power they initially held. As such, the US states are now much less like countries and more like provinces of other countries, although they do have possibly more authority than typical political subdivisions.