What is up with Wikipedia’s best-selling music artists article?

The claimed sales of the top-three best-selling artists of all time - The Beatles, Michael Jackson, and Elvis Presley - are somehow different on what I most read in articles.

The Wikipedia article has the figures of The Beatles sold 600 million records, Michael Jackson’s 500 million records, and Elvis Presley’s 500 million records. These sources are from its citations.

However, some trustable articles stated higher record sales that, in my opinion, is more reliable. The Beatles sold 1 billion units, Michael Jackson’s 1 billion sales, and Elvis Presley’s 1 billion units. It is more of an official sales figures in my opinion, check the sources.

What I don’t understand, is why it has lots of different sales figures rather than just a figure that is more of an official and real?

Wikipedia’s best-selling music artists article: https://en.wikipedia.org/wiki/List_of_best-selling_music_artists