Could have been a sloppy OS build and install process on Dell's part. Or, I wonder if it could be because the build/version of Windows 10 that came on your XPS 8900s might have been pre-November update, and that November update was applied after you received your desktops? My experience is that when I applied the November Windows 10 update, taking my Windows 10 to build 10586/version 1511, the update left behind both the folders you mentioned. I had deleted them previously after successfully completing upgrade from Windows 7 to Windows 10. Or, Dell, rather than you, might have applied the November update to their baseline OS, left those folders and applied that image when building your desktops (again, sloppy work on Dell's part).  Just some thoughts. Good luck.
On my new XPS 8900 desktop, I find a Windows.old folder, which is almost 20 GB in size.  Windows 10 was pre-installed. Is this a left-over from a Windows 8(.1) installation? How can I check that, and how can I get rid of the Windows.old folder?
XPS 8900 why do I have a Windows.old folder