If the phone was manufactured in the past month or so, it could be possible that it runs on 13.3 right out of the box and it’s not a bad thing. I would start worrying about the battery if it’s still runs on 13.1.
Nothing to worry about. That’s actually great! You don’t need to restore to get the latest software and you can just start using it. Just inspect the device for any physical flaws. Battery may drain a bit quicker after first use if you sync via iCloud.
My work iPhone 11 and my personal iPhone 11 Pro Max both came with iOS 13.3. They were still factory shrink wrapped when I received them. I did not but either at launch, so I bought after 13.3 was released.
I would not worry about it. It is still likely new.