It's almost always marketing and some stupid idea someone there had. I don't know why non-technical people try and claim so much ownership over versioning. You nearly always end up with these ridiculous outcomes.
"I know! Let's restart the version numbering for no good reason!" becomes DOOM (2016), Mortal Kombat 1 (2025), Battlefield 1 (2016), Xbox One (not to be confused with the original Xbox 1)
As another example, look at how much of a trainwreck USB 3 has become
Or how Nvidia restarted Geforce card numbering
Xbox should be in the hall of fame for terrible names.
There's also Xbox One X, which is not in the X series. Did I say that right? Playstation got the version numbers right. I couldn't make names as incomprehensible as Xbox if I tried.