Dell is throwing a curve ball this year to how it is naming its 2019 XPS 15. To the average consumer, however, the change may be a little confusing. The new 7xxx series will directly succeed the older 9xxx series even though the first digit is lower in order to align the XPS series with the 7xxx Inspiron, Vostro, Latitude, and Precision families.
https://www.notebookcheck.net/Why-is-it-called-the-XPS-15-7590-and-not-the-XPS-15-9580-Dell-explains.422276.0.html