Have you migrated from a backup that had other users? Or when deleting users on that previous machine, have you selected to “keep” the data ?
Otherwise this is unlikely to happen….
Go to Users and delete users. This time you should ensure to select that option to also delete data (i don’t rememver how this is called/proposed, but it is obvious). Now saying this, i know that you said there are no (at least visible) users….
This is one of the reasons I usually prefer not to use automated migration when switching machines…. In my case, over time my other family users have their own macs and i realised there was no reason for me to keep their profiles and data. Performing manual migration (using external drive, copy all data user by user with all their respective folders (documents, images, video…) allow you then to transfer appropriate data to appropriate mac as needed and above all, start fresh without crowding your (in your case very limited) space with stuff (data or apps) you no longer need. Now i know the counter arguments to this is about 1) knowing what you are doing and 2) yes it will be longer to reinstall the apps while drinking coffee (although that additional time will also be compensated with “saving time” that otherwise you would loose with all the older app versions that you will nevertheless need to update 🤷🏽♂️)
Hope these ideas help.