this is normal behavior.
All computers that don´t match one of the sorting-criteria for your other groups are automnatically sorted into the lost&found folder.
The groups that get automatically created are dependend on the Workgroup / Domainname of the system.
If later on a system gets sorted into a different folder the old folder with the Workgroup / Domainname still persists and won´t delete itself automatically.
So we can sure say there was a system communicating with your ePO not matching on of your sorting-criteria and havin a not known Workgroup / Domainname (for example "local", "Workgroup" or stuff like that).
Thaks for the update...
Thanks & Regards