For the most part, the Dell XPS series has been easy to follow. The 13.3-inch XPS 13 9343 was succeeded by the 9350 followed by the 9360, 9370, and 9380. Similarly, the 15.6-inch XPS 15 series consists of the 9530, 9550, 9560, 9570, and so forth.
Unfortunately, Dell threw this convention all out the window early last year with the launch of the XPS 13 7390 and XPS 15 7590 to directly supplant the XPS 13 9380 and XPS 15 9570, respectively. Their reasoning was understandable at the time: the naming convention for the Inspiron, Latitude, Vostro, and Precision families topped out at the 7xxx series meaning that the XPS family was the odd one out with its 9xxx series model names. Thus, Dell dropped the XPS 9xxx name in favor of the XPS 7xxx to be more in line with the product families.
In hindsight, the move opened a can of worms with existing XPS fans who have grown accustomed to the 9000 series naming convention. Users shopping for an XPS 13, for example, may have seen the XPS 13 7390 as an older model when compared to the XPS 13 9380 even though the opposite is true simply because the first digit of the newer model is a lower number. A Dell representative called the XPS naming change a "mistake" as it caused confusion for both enthusiasts and shoppers.
Dell will fortunately move back to the original 9000 series naming scheme starting with the 2020 XPS 13 9300. Future revisions will hopefully be called the 2021 XPS 13 9310, 2022 XPS 13 9320, and so forth. The latest XPS 13 will be a fresh start for the venerable series as even the logo will be redesigned. We can only assume that the next eventual XPS 15 refresh will be called the XPS 15 9500 unless if Dell decides to have another facepalm moment.