Account Unknown has full Security permissions on my own User - huh?

Account Unknown(S-1-5-21-1345748419-2493160549-2943627078-1000)
has full permissions in the "Advanced Security Settings" for my own user account on my Win 10 Pro 64-bit PC.
What is it? Is this normal?

If I go to C:\Users\ , it lists four users:
[Myself]
Default
Default.migrated
Public.

If I right-click on [Myself] and hit Properties, then go to the security tab, it lists four "Group or user names:" with certain Permissions:
SYSTEM with full permissions - all items checked
Account Unknown(S-1-5-21-1345748419-2493160549-2943627078-1000) with the same full permissions - all items checked
[Myself] with the same full permissions - all items checked
Administrators with the same full permissions - all items checked

What is the Account Unknown(S-1-5-21-1345748419-2493160549-2943627078-1000), and how did it get there?

Two clues:
1) My PC is dual-booting Win 7 Pro 64-bit and this Win 10 Pro 64-bit. I'm in the Win 10
2) 15 minutes ago (and before checking this), I turned on Public file sharing so other PCs on my home network could see any documents I put in those Public folders or maybe vice versa. I do NOT have "Homegroup" turned on.

But wha' happening here?

Brink - thanks for suggestion.
But I'm not sure about this at all.
By the way, if I open a cmd and do the wmic (or wimc?) command to show all the accounts, this unknown one does not appear!
What's the worst that could happen if I delete this unknown account from the security tab?

If it's not listed in the Registry, it may also be from a previous installation.

Edwin - good catch. The Account Unknown SID does NOT appear under HKEY_USERS in Win 10.

HOWEVER, as I said, my PC is dual-booting Win 7 Pro 64-bit and Win 10 Pro 64-bit. Guess what? My Account Unkown SID DOES appear under HKEY_USERS if I reboot into Win 7 !!! And that SID is my own user in Win 7 !!!

Now, in Win 7, I have two different Account Unknowns. I betcha they are from my Win 10 (and maybe its predecessor, my old Win 8/8.1.)

Stay tuned.

Thanks to you both for helping me find this.

Brink - yes, that makes sense.

Edwin said:

If it's not listed in the Registry, it may also be from a previous installation.

and if its listed in the registry like in my case then what?